IRLF 


T  A 


EXCHANGE 


FOUNDATIONS 

FOR 

BRIDGES  AND  BUILDINGS 


BY 

ROLAND  P.  DAVIS,  S.  B.,  M.  C.  E. 


A  THESIS 


PRESENTED 

TO  THE 
FACULTY  OF  THE  GRADUATE  SCHOOL 

OF 
CORNELL  UNIVERSITY 

FOR  THE  DEGREE  OF 
DOCTOR  OF  PHILOSOPHY 


1914 


FOUNDATIONS 

FOR 

BRIDGES  AND  BUILDINGS 


BY 

ROLAND  P.  DAVIS,  S.  B.,  M.  C.  E. 


A  THESIS 


PRESENTED 
TO  THE 

FACULTY  OF  THE  GRADUATE  SCHOOL 

OF 

CORNELL  UNIVERSITY 

FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


1914 


Reprinted  by  Permission 
From 

FOUNDATIONS  OF  BRIDGES  AND  BUILDINGS 

By 
HENRY  S.  JACOBY  and  ROLAND  P.  DAVIS 


Published  by 

McGRAW-HILL  BOOK  COMPANY,  Inc. 
239  West  39th  Street,  New  York 

Copyright,  1914,  by  McGraw-Hill  Book  Co.,  Inc. 


PREFACE 

In  writing  the  chapter  on  cofferdams  an  attempt  has  been 
made  to  classify,  give  examples  of,  and  to  point  out  the  ad- 
vantages of,  the  different  types  of  cofferdams  placed  under 
various  conditions.  In  an  art  which  is  based  almost  entirely 
upon  precedent  there  is  often  too  marked  a  tendency  to  copy 
methods  used  by  others  without  carefully  analyzing  the  condi- 
itons  to  see  what  type  is  best  adapted  in  any  given  instance. 
It  is  true  that  in  many  instances  any  particular  type  has  but 
few  advantages  over  some  other  type,  but,  nevertheless,  given 
certain  physical  conditions  obtaining  at  the  site,  there  is  always 
some  one  type  of  cofferdam  which  will  prove  most  satisfactory 
and  economical. 

The  pneumatic  caisson  process  of  placing  foundations  affords 
an  example  of  the  value  of  applying  scientific  principles  in  the 
development  of  an  art.  The  wooden  type  of  caisson  is  the 
most  widely  used  in  this  country  and  for  this  reason  more  space 
has  been  devoted  to  it  than  to  the  other  types. 

One  of  the  problems  largely  unsolved  as  yet  is  the  question 
of  caisson  disease.  The  two  articles  on  this  subject  give  a 
careful  resume  of  our  present  knowledge  of  this  subject. 

Because  of  the  relatively  low  cost  of  shallow  footings  the 
subject  of  spread  foundations  is  an  important  one.  In  the 
chapter  on  this  subject  all  types  which  have  proven  successful 
and  economical  are  described.  Although  any  rational  analysis 
of  the  stresses  in  a  spread  footing  must  be  more  or  less  approxi- 
mate, yet  only  through  such  a  design,  coupled  with  good  judg- 
ment, can  an  economical  footing  be  developed. 


399561 


TABLE  OF  CONTENTS 


CHAPTER  VI.— COFFERDAMS 

Art.  64.  The  Cofferdam  Process 

65.  Earth  Cofferdams 

66.  Wooden     Sheet-Pile    Coffer- 

dams 

67.  Single  Wall  with  Guide  Piles 

68.  Sheet    Piling    Supported    by 

Frames 

69.  Sheet    Piling    Supported    by 

Cribs 

70.  Steel  Sheet-Pile  Cofferdams 

71.  Self-Supporting  Steel   Sheet- 

Pile  Cofferdams 

72.  Crib  Cofferdams 

73.  Movable  Cofferdams 

74.  Miscellaneous  Types 

75.  Puddle  and  Leakage 

76.  Design  of  Cofferdams 

77.  Choice  of  Type 

CHAPTER  VIII.— PNEUMATIC  CAIS- 
SONS FOR  BRIDGES 

Art.  92.  The  Pneumatic  Process 

93.  Caisson  Roof  Construction 

94.  Sides  of  Working  Chamber 

95.  Details  of  Cutting  Edge 

96.  Bracing  of  Caisson 

97.  Crib  Construction 

98.  Cofferdam  Construction 

99.  Pneumatic  Caissons  of  Con- 

crete 

100.  Pneumatic  Caissons  of  Metal 

101.  Cylinder  Pier  Caissons 

102.  Combination   Cylinder   Cais- 

sons 


CHAPTER  IX.— PNEUMATIC 
CAISSONS  FOR  BRIDGES  (CoN.). 

103.  Shafts  and  Air-Locks 

104.  Design  of  Caissons 

105.  Building     and     Placing     the 

Caisson 

106.  Sinking  the  Caisson 

107.  Removing  Spoil  from  Work- 

ing Chamber 

108.  Concreting  the  Air  Chamber 

109.  Rate  of  Sinking 
no.  Frictional  Resistance 

in.  Physiological  Effects  of  Com- 
pressed Air 
112.  Prevention  of  Caisson  Disease 

CHAPTER  XV.— SPREAD 
FOUNDATIONS 

Art.  149.  General  Considerations 

150.  Early  Types  of  Footings 

151.  Modern    Types    of    Spread 

Foundations 

152.  Construction      of      I-Beam 

Grillage 

153.  Design  of  I-Beam  Grillage 

154.  Design    of    Double-Column 

Footings 

155.  Distribution  of  Pressure  on 

Base 

156.  Steel  Grillage  foundations 

157.  Design   of   Reinforced-Con- 

crete  Spread  Foundations 

158.  Design   of   Reinforced-Con- 

crete  Column  Footings 

159.  Concrete    Spread    Founda- 

tions 


ART.  63  DESIGN   OF   SHEET-PILING  197 

the  section.  The  values  of  the  section  modulus  for  the  com- 
mercial sizes  of  steel  sheet  piles  may  be  obtained  from  the  manu- 
facturers. The  corresponding  width  to  be  used  in  computing 
the  bending  moment  per  pile  is  the  distance  center  to  center  of 
interlock  when  assembled. 

The  design  of  sheet-piling  to  resist  earth  pressure  in  which 
the  material  has  more  or  less  cohesion  is  not  on  a  basis  that  is 
entirely  satisfactory.  The  conditions  vary  so  widely  and  often 
the  material  penetrated  in  any  locality  occurs  in  layers  of 
different  density  or  character  that  it  is  well  to  make  the  design 
so  as  to  be  on  the  safe  side.  Some  engineers  design  all  sheet- 
piling  for  hydrostatic  pressure,  increased  by  50  percent  or  more 
for  wet  slippery  material. 


CHAPTER  VI 
COFFERDAMS 

ART.  64.     THE  COFFERDAM  PROCESS 

When,  for  some  purpose,  it  is  desired  to  exclude  the  water 
and  expose  a  portion  of  the  bottom  of  a  river,  lake,  or  other 
body  of  water,  a  structure  called  a  cofferdam  is  employed. 
This  cofferdam  is  a  temporary  structure,  practically  water- 
tight and  large  enough  to  provide  adequate  room  for  working. 

Denned,  a  cofferdam  is  a  temporary  structure  used  for  the 
purpose  of  excluding  the  water  from  a  given  site,  or  area,  either 
wholly  or  to  such  a  degree  that  with  a  reasonable  amount  of 
pumping  the  permanent  substructure  may  be  built  within  it 
in  the  open  air,  or  that  other  work  may  be  accomplished. 

The  building  of  the  permanent  substructure  may  include 
pile  driving,  placing  grillages,  building  piers  and  abutments, 
etc.,  while  other  work  may  include  the  construction  of  dams, 
removal  of  sunken  vessels,  etc.  Where  the  ground  is  satu- 
rated with  water,  cofferdams  are  sometimes  used  in  placing 
foundations  for  buildings. 

Cofferdams  are  usually  built  in  place.  They  may  be  self- 
contained  or  may  depend  for  strength  on  the  natural  bottom, 
as  is  the  case  where  guide  piles  are  used.  Bracing  may  be  used 
to  resist  the  lateral  pressure  against  the  walls. 

To  obtain  water-tightness  the  sides  of  the  cofferdam  must 
be  tight  and  the  soil  on  which  the  cofferdam  rests  must  be 
impervious.  If  the  latter  condition  does  not  exist,  either 
the  sides  of  the  cofferdam  must  extend  through  the  pervious 
material  to  an  impervious  stratum  or  else  a  layer  of  concrete 
must  be  spread  over  the  bottom  inside  the  cofferdam  and 
allowed  to  harden  before  pumping  is  begun.  Absolute  water- 
tightness  is  seldom  sought,  it  being  cheaper  to  pump  a  moderate 

198 


ART.  65  EARTH   COFFERDAMS  1 99 

amount  of  leakage  than  to  go  to  the  heavy  expense  of  building 
a  structure  that  will  not  leak.  The  cofferdam  should  be  so 
designed  that  the  combined  cost  of  construction,  maintenance 
and  pumping  shall  be  a  minimum. 

To  depths  of  from  20  to  30  feet  the  cofferdam  process  will 
prove  the  best  and  cheapest  method  of  founding  bridge  piers 
and  abutments,  but  for  depths  greater  than  30  feet,  owing  to  the 
difficulty  of  properly  bracing  the  cofferdam  against  the  pressure 
of  the  water,  as  well  as  preventing  heavy  leakage,  some  other 
method  is  usually  preferable.  Cofferdams  over  50  feet  deep 
have  been  used  in  a  few  instances. 

Cofferdams  may  be  constructed  of  earth,  timber,  steel  or 
concrete.  They  may  be  divided  into  five  general  classes:  earth, 
sheet  pile,  crib,  movable  and  miscellaneous  cofferdams.  These 
classes  will  be  described  separately  in  the  following  articles. 

ART.  65.     EARTH  COFFERDAMS 

Of  the  five  classes  the  earth  cofferdam  is  the  oldest  in  origin 
and  simplest  in  construction.  Its  use  is  usually  limited  to 
shallow  water  with  low  velocities  of  current.  It  is  made  of  a 
bank  of  earth  placed  around  the  site  to  be  enclosed,  and  of  a 
thickness  sufficient  to  furnish  the  required  stability  and  to 
keep  the  leakage  down  to  a  small  amount.  The  earth  bank 
should  be  carried  up  2  or  3  feet  above  the  water-level  with  a 
width  of  at  least  3  feet  at  the  top,  and  with  side  slopes  corre- 
sponding to  the  natural  slope  of  the  material.  The  embankment 
should  preferably  be  composed  of  a  mixture  of  clay  and  sand  or 
gravel,  but  if  clay  is  scarce  the  bank  may  be  composed  of  sand 
with  a  clay  wall  in  the  center. 

The  amount  of  embankment  may  be  somewhat  reduced  by 
using  one  or  two  rows  of  sheet-piling,  in  which  case  the  cofferdam 
may  resemble  more  or  less  closely  the  sheet-pile  cofferdam 
described  in  later  articles.  As  to  whether  in  any  given  case  the 
cofferdam  should  be  classed  as  an  earth  or  sheet-pile  cofferdam 
will  depend  upon  whether  or  not  stability  and  water-tightness 
depend  primarily  upon  the  earth  filling. 


2OO  COFFERDAMS  CHAP.  VI 

Where  the  depth  of  water  is  not  more  than  4  or  5  feet  and  the 
velocity  of  the  current  would  wash  away  loose  material,  coffer- 
dams may  be  made  of  ordinary  canvas  bags  about  half  filled 
with  a  mixture  of  clay  and  sand.  It  is  important  that  the  bags 
shall  be  but  partially  filled  for  otherwise  they  will  not  pack 
together  closely. 

A  modern  and  up-to-date  use  of  the  earth  cofferdam  is  found 
in  the  construction  of  the  cofferdams  of  the  West  Neebish 
Channel  of  the  St.  Mary's  River.  In  some  places  the  depth  of 
the  water  was  far  too  great  for  the  economical  use  of  earth 
cofferdams  and  was  justified  here  only  by  the  extremely  favor- 
able conditions  that  obtained  for  placing  the  earth.  Two  sub- 
sidiary cofferdams  were  first  constructed  across  the  channel 
about  midway  between  the  main  ones  in  order  to  stop  the 
current  and  divert  the  flow  to  another  course.  *" These  tem- 
porary dams  were  about  1000  feet  apart  at  the  site  of  the 
channel  and  extended  across  the  river  from  the  mainland  to 
the  island,  varying  in  direction  to  suit  the  contours  of  the  river 
bed.  They  were  built  in  2  to  7  feet  of  water  flowing  3  to  6 
miles  an  hour.  The  construction  of  these  dams  stopped  the 
flow  of  water  in  the  West  Neebish  Channel  of  the  river,  that  the 
main  cofferdams  could  be  built  in  still  water,  and  also  laid  bare 
a  part  of  the  site  of  the  channel  about  1000  feet  long.  In 
building  these  temporary  dams,  which  varied  from  4  to  10  feet 
in  height,  broken  stone  and  rock  were  dumped  from  scows  on 
the  line  of  the  dams  until  the  force  of  the  current  was  broken 
and  the  rock  fill  carried  above  the  water.  Sandy  clay  was 
then  brought  in  and  dumped  on  the  upstream  side  of  these 
rock  embankments  in  order  to  silt  up  the  openings  and  pro- 
duce water-tight  dams." 

The  main  cofferdams  which  unwatered  the  86oo-foot  section 
of  the  work  were  structures  of  unusual  size.  The  upstream 
cofferdam  was  1900  feet  long  and  was  built  in  water  from  2  to 
18  feet  in  depth.  laThis  cofferdam  has  a  minimum  width  of 
8  feet  at  the  top,  which  is  7  feet  above  the  water,  and  has  side 
slopes  on  the  water  side  of  about  i  on  ij,  and  of  about  i  on 

1  Engineering  Record,  vol.  56,  page  112,  Aug.  3,  1907. 


ART.  65 


EARTH   COFFERDAMS 


2OI 


2  on  the  other  side.  The  other  main  cofferdam  is  8600  feet 
downstream  from  this  one.  It  has  a  total  length  of  2600  feet, 
and  in  plan  is  arched  slightly  downstream  against  the  water  on 
that  side  of  it.  This  cofferdam  was  built  in  water  from  nothing 
to  26  feet  deep;  it  has  a  minimum  width  of  12  feet  at  the  top, 
which  is  6  feet  above  the  water;  its  side  toward  the  water  is 
built  on  an  average  slope  of  i  on  2,  and  the  one  on  the  other 
side  of  i  on  2\. 

"The  construction  of  the  upstream  main  cofferdam  was 
started  soon  after  the  current  of  the  river  had  been  broken  by 
the  temporary  dams.  Sandy  clay  and  mud  excavated  by  the 


EL  3452*. 


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rAbovt  ISO'  to 
Bottom  of  Excavation 

Cross  Section 

•     ' 

FIG.  650. — Sheeting  for  Earth  Cofferdam  on  the  Ohio  River. 

dredges  at  work  on  the  adjacent  sections  of  the  channel  were 
brought  to  the  site  in  bottom-dump  scows  and  deposited  in 
place.  When  the  banks  thus  formed  had  been  carried  up 
until  the  bottom-Pump  scows  would  operate  no  longer,  the 
materials  were  loaded  on  flat-deck  scows,  and  handled  from 
these  to  place  in  the  embankment  by  a  clam-shell  bucket  on 
a  derrick  scow." 

French  engineers  have  made  extensive  use  of  the  earth 
cofferdam  for  work  on  their  various  canals.  In  some  work  on 
the  Meuse  Canal,  described  in  Annales  des  Fonts  et  Chaussees, 


2O2 


COFFERDAMS 


CHAP.  VI 


1896,  page  539,  gravel  ranging  in  size  from  about  i  to  4  inches — 
being  the  residue,  after  the  sand  was  used,  of  material  dredged 
from  the  canal  bottom — was  employed.  Water-tightness 
was  obtained  by  placing  a  layer  of  tan-bark  over  the  water 
face.  In  some  cases  the  head  of  water  on  the  cofferdam  was 
as  much  as  9  feet. 

In  place  of  earth,  cofferdams  are  sometimes  made  of  fascines. 
The  cofferdam  for  a  concrete  dam  at  Milford,  Conn.,  was  made 
by  forming  brush  into  mats,  which  were  sunk  by  loading  with 
rocks,  the  layers  of  brush  and  stone  alternating.  To  give  water- 
tightness  a  layer  of  earth  was  placed  over  the  upstream  side. 

Fig.  650  illustrates  the  cross-section  of  the  earth  cofferdam 
with  sheeting  used  in  the  construction  of  the  Ohio  River  Lock 


Held  vertically  ty  Otrrx*  Boat 
S. 


™^ 


FIG.  65  b.  —  Method  of  Constructing  Ohio  River  Cofferdam. 

and  Dam  48,  where  the  bottom  was  composed  of  sand.  To 
break  the  current  a  line  of  sheet-piling  was  first  driven.  Frames 
were  then  placed  by  a  boat  as  shown  in  Fig.  65^  and  connected 
to  the  sheet-piling.  Vertical  planking  was  placed  against  the 
frames  and  the  interior  then  filled  wirh  dredged  material. 
Gravel  was  placed  along  the  outside  of  the  sheet-piling  up  to  its 
top  and  on  a  slope  of  about  45  degrees;  the  space  between  the 
sheet-piling  and  sheeting  was  also  filled  with  earth,  and  finally 
sand  was  placed  against  the  inside  wall  of  sheeting  up  to  the 
elevation  of  the  sheet-piling  tops.  This  sand  had  a  very  gentle 
slope,  running  approximately  100  feet  before  reaching  the  eleva- 
tion of  the  bottom  of  the  sheeting. 


ART.  66  WOODEN   SHEET-PILE   COFFERDAMS  203 

ART.  66.    WOODEN  SHEET-PILE  COFFERDAMS 

The  sheet-pile  type  may  be  considered  as  the  standard  form 
of  cofferdam.  It  consists  of  rows  of  sheet-piling,  usually  not 
more  than  two,  extending  around  the  site  to  be  enclosed.  The 
piling  is  held  in  place  in  various  ways  as  described  in  the  fol- 
lowing articles.  The  sheet-piling  serves  the  function  of  giving 
water-tightness  to  the  structure,  and  to  this  end  some  form 
ofj  intermeshing  or  interlocking  piling  is  always  employed. 
Strength  to  resist  the  pressure  of  the  water  outside  is  furnished 
by  guide  piles,  frames,  or  cribs,  in  addition  to  a  large  amount 
of  internal  bracing.  The  sheet-piling  may  be  of  wood  or  steel; 
at  the  present  time  (1914)  the  use  of  various  forms  of  steel  piling 
is  rapidly  increasing. 

Where  it  is  possible  to  drive  piles  some  distance  into  the 
soil  the  sheet-piling  is  best  supported  by  vertical  guide  piles 
and  horizontal  wales.  The  latter  will  not  only  furnish  a  guide 
for  the  sheet-piling  while  being  driven  but  will  also  add  strength 
to  the  cofferdam,  thus  decreasing  the  amount  of  internal  brac- 
ing necessary. 

DOUBLE  WALL  WITH  GUIDE  PILES. — Fig.  66a  shows  the  de- 
tails of  this  type  of  cofferdam.  It  is  composed  of  vertical 
guide  piles,  horizontal  waling  and  cap  timbers,  vertical  sheet 
piles  and  a  puddle  filling.  Rods  are  usually  put  in  near  the 
top  to  connect  each  pair  of  guide  piles  in  order  to  prevent  the 
filling  from  spreading  the  walls  apart.  If  the  top  of  the 
cofferdam  is  but  slightly  above  water-level,  struts  are  often 
placed  near  and  parallel  to  the  tie  rods,  serving  to  hold  the  two 
walls  apart. 

The  bearing  piles  are  driven  more  deeply  into  the  earth  than 
the  sheet  piles,  the  aim  being  to  drive  them  far  enough  to 
develop  the  full  transverse  strength  of  the  pile  when  acting  as 
a  free  cantilever  above  the  earth.  The  sheet-piling  should  be 
driven  to  a  fairly  impervious  stratum  to  prevent  leakage  under 
the  cofferdam.  The  space  between  the  walls  should  be  filled 
with  earth,  preferably  an  intimate  mixture  of  sand  and  clay  or 
gravel  and  clay,  to  form  a  puddle  (Art.  75),  which  will  mate- 


204  COFFERDAMS 

&  "x  12  "JheefPile.,  ..Strut  10  x  10 ' 

Drift  Bolt-  — 


CHAP.  VI 


.-Cut  Washer 


-Wale,  10x10' 


//////////A//*/, 
'////////,W'M 


ww 

Hi 

^11 


wm/'t 

lit 

/'^/// 


FIG.  66a. — Section  of  the  Double  Wall  of  a  Cofferdam  Showing  Puddle  Chamber. 


El.+  llO.O          \ 


\        L  och 


'ft?1"80** '"' ';':  '•'• ' -wina :': ":':-  ''"'•''•  ^'-  ;§: 


•r//^    '  •  ••••.•^:v^. 

EI.-HOO.O 

ENG.  NE.WS. 


Transverse      Section. 

FIG.  666. — Details  of  Double  Wall  of  Sheet-pile  Cofferdam,  Charles  River,  Boston, 

Mass. 


WOODEN    SHEET-PILE    COFFERDAMS 


205 


rially  assist  the  sheet-piling  in  making  the  cofferdam  water- 
tight. This  puddle  should  be  placed  in  thin  layers  and  thor- 
oughly tamped  in  a  damp  state.  Before  placing  the  same  it 
will  usually  be  advisable  to  dredge  out  the  soft  material  on 
the  bottom  to  an  impermeable  stratum.  This  puddle  filling, 
in  addition  to  promoting  water-tightness,  will  materially 
strengthen  the  structure.  Clay  is  often  banked  around  the 
outside  of  the  cofferdam  to  safeguard  it  further  against  leakage. 

The  cofferdam  should  have  its  puddle  chamber  wide  enough 
to  develop  the  required  strength,  furnish  water-tightness,  and 
afford  sufficient  space  for  plac- 
ing machinery,  gangways,  etc. 
One  rule  for  the  width  of  un- 
braced cofferdams  is  to  make 
it  equal  to  the  height  above  the 
ground  up  to  10  feet,  and  when 
the  height  is  greater  than  this, 
make  the  width  10  feet  plus 
one-third  the  height  in  excess 
of  10  feet.  The  design  of  sheet- 
piling  is  considered  in  Art.  63. 

A  well-designed  cofferdam  of 
the  double-wall  type  was  used 
in  the  construction  of  the  locks 
for  the  Charles  River  Dam, 
Boston,  Mass.,  where  the  length 

was  about  625  feet  and  the  width  about  250  feet,  surround- 
ing an  area  of  approximately  4  acres.  The  maximum  depth  of 
water  on  the  outside  at  low  water  was  20  feet.  As  shown  in 
Fig.  666  and  c,  the  cofferdam  consisted  of  two  rows  of  guide 
piles  ii  feet  apart,  with  piles  spaced  10  feet  on  centers,  which 
through  wales  supported  6-inch  splined  and  grooved  sheet-piling. 

The  guide  piles  were  of  spruce,  45  feet  long,  and  each  alternate 
pile  was  braced  by  a  batter  or  spur  pile.  The  sheet-piling  was 
of  yellow  pine  with  spruce  splines  and  was  38  feet  in  length. 
The  remainder  of  the  details  are  clearly  shown  in  the  diagrams. 
A  filling  of  sand  and  clay  was  placed  around  both  the  inside  and 


FIG.  66c. 


206  COFFERDAMS  CHAP.  VI 

outside  of  the  cofferdam  as  well  as  in  the  puddle  chamber. 
On  the  inside  it  had  a  width  of  25  feet  at  the  top  and  then  sloped 
down  on  a  2  on  i  slope,  thus  making  virtually  a  combination 
pile  and  earth  cofferdam.  Although  probably  not  an  econom- 
ical form  of  cofferdam  for  ordinary  use,  yet  in  a  case  like  this 
where  the  filling  was  permanent  construction,  it  made  an  ad- 
mirable structure  to  withstand  the  37-foot  head,  which  was 
approximately  the  maximum  height  of  high  water  above  the 
bottom  of  the  lock  masonry. 

ART.  67.     SINGLE  WALL  WITH  GUIDE  PILES 

Where  the  space  available  for  the  cofferdam  is  restricted  or 
where  the  area  of  the  site  to  be  enclosed  is  small  and  the  head 
of  water  not  great,  a  cofferdam  having  a  single  wall  is  preferable 
to  the  double-wall  type.  Other  conditions  being  the  same  the 
former  type  will  require  more  bracing  than  the  latter,  but  in 
many  cases  this  will  prove  cheaper  than  the  extra  wall. 

Figs.  67^  and  b  show  the  details  of  cofferdams  used  for  the 
rectangular  and  pivot  piers  for  the  Illinois  Central  Railroad 
bridge  across  the  Tennessee  River  at  Gilberstville,  Ky.,  both  of 
which  are  standard  types  for  single-wall  cofferdams  of  moderate 
size.  Before  placing  these  cofferdams  the  bottom  of  the  river 
was  dredged  down  to  about  17  feet  below  low  water  to  hard 
gravel.  Cofferdam  guide  piles  were  then  driven  and  ioX 
12-inch  wales  bolted  to  the  outside,  after  which  9X1 2-inch 
triple-lap  sheet-piling  was  driven  against  the  latter,  penetrating 
the  gravel  from  4  to  6  feet.  The  piers  were  founded  on  bearing 
piles  driven  from  16  to  20  feet  into  the  gravel  and  cut  off  2 
feet  above  the  bottom  before  the  cofferdam  was  placed. 

Before  pumping  out  the  water  a  3 -foot  layer  of  concrete  was 
placed  on  the  bottom,  thus  preventing  leakage  of  water  beneath 
the  cofferdam;  later  it  served  as  a  cap  for  the  bearing  piles. 
The  bracing,  which  is  clearly  shown  in  the  illustrations,  was 
placed  as  the  water  was  pumped  out.  The  octagonal  cofferdam 
was  braced  by  annular  trusses  which,  by  their  arch-like  action, 
proved  to  be  a  very  rigid  form  of  bracing,  and  yet  offered  no 


ART.  67 


SINGLE    WALL   WITH    GUIDE   PILES 


207 


I  Quarter  Ran  of  Upper 
|  Cofferdam  Section. 


Half  Plan  of 
'Cofferdam  Bracing     i        Quarter   Plan 

'Rod  of 


Cofferdam  for  Center  Pier  No.3 


9Sporces  4 '//* 
•  37'O' 


10  "*I2  '  IO' 


I2'*IO 


<\j  I    n, 


10' 


K)'*  12", 


Half  Plan  of  Lower  Cofferdam  Half  Plan  of  Upper  Cofferdam. 

Cofferdam   for  Piers   $5  and  Q 

FIG.  6ya. — Cofferdams  with  Single  Walls  of  Timber  Sheet  Piling  Supported  by 
Wales  and  Guide  Piles,  for  Piers  of  Illinois  Central  Railroad  Bridge  over  Tennessee 
River,  at  Gilbertsville,  Ky.  See  also  Fig.  6aa. 


208 


COFFERDAMS 


CHAP.  VI 


obstruction  to  the  work  of  building  the  piers,  which  were  of 
concrete.  The  forms  for  these  piers  were  braced  against  the 
trusses. 

A  good  example  of  a  very  large  and  high  single-wall  sheet- 
pile  cofferdam,  very  strongly  braced,  is  illustrated  in  Fig.  ojc, 
this  structure  being  used  to  found  the  pier  of  a  lift  bridge  for 


^aMsp^™ip«5fpp^ 

j  j  1 1  j  |  ]  i !  1 1 J  ]  1 1  ii  ii  i  ii  1 1 1 1  ii  |  i'ij  i  ij  M  1 1 1  n  J  i ; 


Canvas  flap. 


LOwWOter  EL45'O"i 


Section  A-A. 
Rectangular  Cofferdam. 


*ffiy 


Section  B-B. 
Octagonal  Cofferdam. 

FIG.  676. — Elevation  of  Cofferdam  Walls. 

the  Chicago  Terminal  Transfer  Railroad.  Two  sides  of  the 
cofferdam  were  on  land,  one  in  water,  and  the  other  two 
partly  in  water  and  partly  on  land. 

A  row  of  guide  piles,  from  6  to  8  feet  apart  and  40  feet  long, 
were  first  driven.  1 "  Six  tiers  of  inside  and  outside  waling  pieces 
were  bolted  to  these  piles,  and  on  the  land  side  370  6X1 2-inch 

1  Engineering  Record,  vol.  50,  page  636,  Nov.  26,  1904. 


ART.  67 


SINGLE    WALL   WITH    GUIDE    PILES 


209 


sheet  piles  34  feet  long  were  driven  between  the  outer  wales, 
and  6X i2-inch  horizontal  guide  pieces  at  the  surface  of  the 


ground  and  4  feet  below  it.     On  the  water  sides  274  34-foot 
Wakefield  piles  9  inches  thick,  made  of  3Xi2-inch  planks,  were 
driven  in  the  same  manner. 
14 


210  COFFERDAMS  CHAP.  VI 

"The  piles  were  driven  as  the  excavation  progressed  inside  of 
the  cofferdam,  and  at  the  same  time  rows  of  transverse  and 
longitudinal  i2Xi2-inch  horizontal  braces,  about  6  and  8  feet 
apart  on  centers  and  from  4  to  6  feet  apart  vertically,  were  set 
with  their  ends  engaging  the  round  piles  on  the  center  lines  of  the 
walls.  At  intersections  these  braces  were  supported  on  8X8- 
inch  vertical  timbers;  one  of  them  was  continuous  and  the  other 
was  cut  to  clear  it,  with  the  square  ends  abutting  against  the 
sides  of  the  first  piece  and  spliced  across  it  with  two  side  fish 
plates.  .  .  .  The  inside  wales  were  of  i2Xi2-inch  timber 
(except  in  the  upper  two  tiers,  where  8X  i6-inch  timber  was  used 
because  it  was  conveniently  available  from  the  contractor's 
stock),  all  of  them  being  lapped  and  halved  at  intersections. 
The  outside  wales  were  uniformly  6X12  inches.  The  round  pile 
caps  and  the  two  upper  rows  of  wales  on  the  water  side  were 
made  of  pX  i3~inch  timber.  All  wales  were  bolted  through  the 
round  piles,  and  the  oblique  joint  in  the  Wakefield  piling  was 
tied  by  bolts  through  both  faces. 

"In  the  longest  dimension  of  the  cofferdam,  the  six  tiers  of 
horizontal  struts  in  each  longitudinal  line  were  divided  into 
seven  panels  by  the  vertical  posts  supporting  them  at  the 
intersections  of  alternate  transverse  braces.  Each  panel  thus 
formed  on  three  of  the  long  lines  and  one  short  line  was 
X-braced  with  2Xio-inch  planks,  spiked  to  the  longitudinal 
struts  at  all  intersections  and  overlapping  in  the  centers  of  the 
panels,  as  shown  in  the  longitudinal  sectional  elevation.  Six 
lines  of  similar  bracing  were  provided  for  the  transverse  struts, 
but  varied  from  that  in  the  longitudinal  direction  in  that  the 
upper  and  lower  pieces  of  the  bracing  overlapped  each  other 
by  the  width  of  the  space  between  two  transverse  struts,  thus 
increasing  the  amount  of  bracing  and  the  rigidity  at  a  point  half 
way  between  the  top  and  bottom  of  the  cofferdam." 

ART.  68.     SHEET-PILING  SUPPORTED  BY  FRAMES 

Where  the  nature  of  the  bottom  is  such  that  piles  cannot 
penetrate  the  same  it  is  necessary  to  employ  a  frame  to  hold  the 


ART.  68 


SHEET-PILING   SUPPORTED   BY   FRAMES 


211 


sheet-piling  in  place.  These  frames  are  usually  built  on  shore, 
floated  to  the  site,  and  sunk.  Where  piers  are  to  be  built  under 
an  existing  bridge  it  is  sometimes  possible  to  suspend  the  frame 
from  the  bridge. 

SINGLE -WALL  TYPE. — At   the    site  of    the  bridge  piers  of 
the  Chicago,  Milwaukee  &  St.  Paul  Ry.  near  Kilbourn,  Wis., 


-19' 6" ~>J 

i  ^  i 


Sec+ional      Side     Elevation. 


FIG.  68a. — Cofferdam  for  Pier  of  Chicago,    Milwaukee,    and  St.   Paul    Railway, 

Kilbourn,  Wis. 

only  a  few  feet  of  sand  covered  the  rock  bottom  on  which  the 
piers  were  to  rest.  As  the  channel  was  narrow  and  the  current 
swift  it  was  essential  that  the  current  be  obstructed  as  little  as 
possible,  and  for  this  reason  the  single-wall  type  was  chosen  in 
preference  to  that  having  a  double  wall.  On  account  of  the 
slight  depth  of  sand,  guide  piles  could  not  be  used  and  so 


212  COFFERDAMS  CHAP.  VI 

recourse  was  had  to  a  frame.  As  shown  in  Fig.  68  a,  the  coffer- 
dam had  V-shaped  ends  to  diminish  the  force  of  the  current 
against  the  structure  and  was  held  in  place  by  wire  guys  an- 
chored to  the  rocks  on  the  sides  of  the  river.  The  frame,  the 
details  of  which  are  shown  in  the  illustration,  was  sunk  by  weight- 
ing with  scrap  rails.  The  covering  consisted  of  9Xi2-inch 
Wakefield  sheet-piling;  in  driving  this  piling  care  was  taken 
to  broom  the  lower  ends  to  give  a  close  fit  to  the  irregular  rock 
surface. 

To  aid  in  giving  water- tightness  to  the  structure  canvas  was 
placed  arOund  the  outside  of  the  cofferdam,  and  was  so  arranged 
that  the  lower  part  rested  flat  on  the  river  bed  for  a  distance  of 
8  feet  out  from  the  dam,  while  the  upper  part  extended  above 
water-level.  The  lower  part  of  the  canvas  was  first  weighted 
down  with  iron  rails  and  sand  bags  to  make  it  fit  closely,  after 
which  about  fifty  car  loads  of  gravel  were  placed  upon  it.  As 
the  water  was  pumped  out  the  structure  was  thoroughly  braced 
as  shown,  but  on  building  the  pier  this  bracing  was  removed 
and  the  cofferdam  walls  braced  against  the  pier. 

One  of  the  largest  and  highest  cofferdams  ever  built  of  wood 
was  of  the  single-wall  sheet-pile-on-frame  type,  and  was  used 
for  the  Mare  Island  Dry  Dock  No.  2.  For  a  complete  descrip- 
tion of  this  structure  see  Engineering  Record,  vol.  57,  page  428, 
April  4,  1908. 

The  cofferdam  was  approximately  150  by  800  feet  in  plan  and 
the  maximum  head  of  water  on  it  was  48  feet.  The  framework 
and  bracing  consisted  of  five  horizontal  courses  of  transverse 
and  longitudinal  timbers,  the  timbers  of  each  course  being  con- 
nected to  those  of  the  adjacent  courses  by  posts,  the  whole  struc- 
ture being  built  as  one  unit  which  rested  on  bearing  piles 
previously  driven  and  sawed  off  under  water.  These  longitudi- 
nal and  transverse  rows  were  12  feet  apart  on  centers.  In  the 
bottom  course  all  timbers  were  16X16  inches  in  section,  while 
those  in  the  next  two  courses  were  14X14  inches,  with  12  X  12- 
inch  timbers  for  the  two  upper  courses.  The  rangers,  i.e., 
the  horizontal  pieces  forming  the  frame  proper  which  holds  the 
sheet-piling  in  position,  were  20X24  inches  in  section  for  the 


ART.  68  SHEET-PILING   SUPPORTED   BY   FRAMES  213 

bottom  course  and  12X12  inches  for  the  top  course,  the  other 
courses  having  intermediate  sizes  between  these  limits.  The 
distance  between  courses  was  approximately  10  feet.  In  addi- 
tion to  the  members  mentioned,  a  large  amount  of  bracing  in 
both  horizontal  and  vertical  planes  was  used. 

The  sheet-piling  units  were  formed  of  two  1 2  X 1 2-inch  timbers 
fastened  together  side  by  side  and  were  60  feet  long,  this  length 
being  obtained  by  using  two  pieces,  one  34  and  the  other  26 
feet  long.  A  tongue-and-groove  joint  was  made  by  spiking  to 
each  piece  of  piling  three  3X4-inch  sticks,  two  on  one  side  and 
one  on  the  other,  thus  making  each  piling  unit  30  inches  wide. 
To  give  additional  water-tightness  to  the  cofferdam  a  large 
amount  of  filling  was  banked  around  the  outside. 

DOUBLE -WALL  TYPE. — This  is  a  form  but  little  used  since  it 
offers  but  slight  advantages  over  the  single-wall  type  and  is  con- 
siderably more  expensive.  It  is  more  easily  made  water-tight 
than  the  single- wall  form,  but  on  the  other  hand,  it  is  very  little 
stronger  because  strength  is  almost  entirely  dependent  on  the 
amount  of  internal  bracing  used.  Where  strength  must  be 
obtained  without  the  use  of  bracing  the  type  described  in 
Art.  69  should  be  used. 

The  cofferdams  for  one  of  the  piers  of  the  Chattahoochee 
River  Viaduct  had  an  inside  framework,  39  feet  long  by  15 
feet  wide,  which  was  composed  of  horizontal  frames  of  6X8- 
inch  pine  timber  braced  with  one  set  of  longitudinal  and  two 
sets  of  transverse  timbers.  These  frames  were  spaced  from  2 
feet  center  to  center  on  the  bottom  to  3  feet  centers  at  the  top 
and  were  held  in  place  by  vertical  posts  between  them,  the  total 
height  of  the  framework  being  9  feet.  The  outside  frames 
were  sufficiently  large  for  a  4-foot  thickness  of  puddle  and 
were  connected  to  the  inside  frames  by  braces  and  rods.  The 
framing  was  partly  built  .on  shore,  launched,  floated  to  place 
and  there  completed. 

The  bottom  of  the  river  had  a  seamy  ledge  covered  with  a 
layer  of  sand  varying  in  depth  from  6  inches  to  3  feet.  As 
soon  as  the  framework  was  sunk  two  rows  of  sheet-piling,  each 
row  consisting  of  a  double  thickness  of  2-inch  pine  plank, 


214  COFFERDAMS  CHAP.  VI 

were  driven,  care  being  taken  to  break  joints.  The  bottom  of 
the  puddle  chamber  was  then  covered  with  two  layers  of  sacks 
loosely  filled  with  sand,  after  which  the  remainder  of  the 
chamber  was  filled  with  clay  puddle.  Considerable  trouble 
was  caused  by  water  coming  up  in  the  cofferdam  through  the 
seamy  ledge  and  this  leakage  was  stopped  only  after  a  2-foot 
layer  of  concrete  was  deposited  through  the  water  and  allowed 
to  harden  before  pumping  out  the  water. 

ART.  69.     SHEET-PILING  SUPPORTED  BY  CRIBS 

For  cofferdams  which  rest  on  hard  bottom  and  are  too  large  to 
employ  internal  bracing  economically,  a  series  of  cribs,  laid  up 
log-house  fashion,  are  used  to  hold  the  sheet-piling  in  place. 
Each  crib  unit  is  made  as  long  as  can  be  conveniently  handled 
and  as  wide  as  is  necessary  to  develop  the  required  stability. 
Rough  logs  are  generally  used  although  in  some  cases  they  may 
be  squared,  but  the  latter  offer  only  a  slight  advantage  over  the 
former.  In  building  these  cribs  the  bottom  courses  are  usually 
started  on  land  and  the  crib  is  built  to  a  height  sufficient  to 
permit  the  top  part  being  well  out  of  water  when  it  is  first 
launched;  after  this  it  is  launched,  floated  to  place  and  com- 
pleted. Where  the  stream  is  low  at  certain  times  of  the  year 
the  cribs  may  sometimes  be  built  in  place.  The  bottom  of  each 
crib  should  be  shaped  to  fit  the  rock  bottom,  and  if  a  few  feet 
of  sand  or  other  material  overlies  the  bedrock  this  should 
be  dredged  out  before  placing  the  cribs.  A  part  of  the  bot- 
tom of  the  crib  is  usually  floored  to  permit  placing  stones  so  as 
to  sink  it. 

After  all  the  cribs  are  sunk  the  remainder  of  the  space  inside 
of  them  may  be  filled  with  stones  or  earth.  The  latter  material 
possesses  the  advantage  of  not  only  giving  the  cribs  great  sta- 
bility but  also  to  secure  water- tightness.  After  the  cribs 
are  placed  sheet  piling  is  driven  around  the  outside  and  banked 
with  earth.  This  type  of  cofferdam  is  very  widely  used  in  build- 
ing dams  for  hydro-electric  plants. 

Fig.  6ga  shows  a  view  of  the  cofferdam  employed  in  the  con- 


ART.  69 


SHEET-PILING   SUPPORTED   BY   CRIBS 


215 


struction  of  a  dam  for  the  Connecticut  River  Power  Co.,  near 
Vernon,  Vt.  The  width  varied  with  the  height  of  the  coffer- 
dam; for  the  upstream  one  the  maximum  width  was  35  feet, 
while  the  maximum  height  was  42  feet,  or  16  feet  above  normal 
water-level.  The  structure  was  of  the  rock-filled  type  made  of 
round  logs  in  y-foot  checks,  with  the  face  logs  slabbed  on  the 
sides  to  give  good  bearing  for  the  sheet-piling.  The  top  of  the 
cribs  were  floored  with  logs  to  serve  as  a  walk  and  also  as  a 
protection  against  ice  pressures.  On  the  outside  the  cribs  were 
sheet-piled  with  3-inch  spline-and-grooved  spruce,  and  this  in 
turn  was  banked  with  earth  up  to  normal  water-level. 


4610 
Section    C-D. 

FIG.  696. — Typical  Section  of  Crib  Cofferdam.     Niagara  Power  Plant,  Electrical 
Development  Company  of  Ontario. 

The  cofferdams  for  the  Niagara  Power  Plant  of  the  Electric 
Development  Co.  of  Ontario  furnish  an  example  of  exceedingly 
strong  and  rigid  cofferdams  placed  under  the  most  trying 
conditions.  In  some  places  the  current  had  a  velocity  as 
high  as  17  feet  per  second  which  made  it  difficult  to  study  the 
nature  of  the  bottom  and  the  depth  of  water  previously  to 
placing  the  cofferdams. 

The  widest  part  of  the  cofferdam  consisted  of  two  lines  of 
parallel,  rock-filled  timber  cribs  with  a  space  between,  sheet- 
piled  and  filled  with  puddle  as  shown  in  Fig.  696.  Both  cribs 


2l6  COFFERDAMS  CHAP.  VI 

were  built  of  squared  timber  with  the  outside  wall  of  the  outer 
crib  laid  solid.  The  width  of  the  cribs  varied  to  meet  the 
variation  in  depth  and  the  bottom  of  the  cribs  was  made  to  fit 
the  irregularities  of  the  rock  surface.  In  shallow  water  the 
cribs  were  built  in  place  but  elsewhere  they  were  constructed 
in  the  river  upstream,  and  by  means  of  cables  from  the  shore 
they  were  floated  into  place  and  were  sunk  by  filling  with  rocks 
the  wells  which  had  bottoms.  For  further  details  of  this  in- 
teresting cofferdam  the  reader  is  referred  to  Engineering  News, 
vol.  54,  page  561,  Nov.  30,  1905. 

ART.  70.     STEEL  SHEET-PILE  COFFERDAMS 

The  advantages  which  steel  sheet-piling  possesses  over  the 
wooden  type  are  discussed  in  Art.  60.  On  account  of  these 
advantages  steel-piling  is  being  used  more  and  more  in  coffer- 
dam work.  The  details  of  the  structures  differ  but  little  from 
those  using  timber  sheet-piling,  the  main  difference  being 
that  the  steel  type,  on  account  of  the  greater  strength  and 
positive  interlock  of  the  piling,  requires  less  bracing. 

Fig.  70^  indicates  a  good  example  of  a  steel  sheet-pile  coffer- 
dam with  guide  piles.  In  the  illustration  the  guide  piles 
and  the  outer  course  of  wales  are  not  shown,  however.  The 
bottom  at  the  site  of  the  pier  consisted  of  hard-pan  to  an  un- 
known depth  covered  with  about  6  inches  of  mud.  The 
depth  of  water  was  about  9  feet  at  mean  tide,  which  had  a 
rise  and  fall  of  about  6  feet.  l"  Round  wooden  piles  were 
driven  8  feet  apart  enclosing  the  site  of  the  83X1 5-foot 
cofferdam;  6Xi2-in  inside  waling  pieces  were  bolted  to  them 
above  high  water. 

" Spacing  blocks  4  inches  thick  and  i2Xi2-inch  inside  wales 
were  bolted  to  the  outside  wales,  forming  guides,  between 
which  were  driven  a  single  row  of  Lackawanna  1 2-inch,  40- 
pound  steel  sheet  piles  35  feet  long.  These  were  all  assembled 
together  before  driving  .  .  .  and  then  driven  ...  by  one 
McKiernan-Terry  steam-hammer  weighing  5000  pounds  and 
making  about  225  strokes  per  minute.  It  was  handled  by  the 

1  Engineering  Record,  vol.  67,  page  268,  March  8,  1913. 


ART.  70 


STEEL    SHEET-PILE    COFFERDAMS 


217 


boom  of  a  floating  derrick  and  went  round  and  round  the  coffer- 
dam, driving  each  pile  a  foot  or  two  at  a  time  until  the  work  was 
completed.  The  driving  was  very  hard,  many  boulders  being 
encountered,  some  of  which  were  displaced  and  others  broken 
by  the  piles.  When  they  could  be  neither  displaced  nor  broken, 
driving  on  the  piles  that  encountered  them  was  discontinued, 
and  adjacent  piles  were  driven  down  to  subgrade  about  6  inches 
below  the  bottom  of  the  footing. 


t™r.ni-. 


I4'n" 


Plan 


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Concrete  Backinb 

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Concrete  Base 

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Section  A-A 


FIG.  700. — Cofferdam  for  Highway  Bridge  Piers  in  Passaic  River,  at  Bridge  St., 

Newark,  N.  J. 

"As  the  bottom  was  excavated  inside  the  cofferdam,  some 
of  the  boulders  which  obstructed  the  sheet  piles  were  left  in 
position  and  the  sides  of  the  excavation  below  them  were 
closed  as  well  as  possible  with  bags  of  cement.  The  cofferdam 
resisted  a  pressure  head  of  about  28  feet  with  very  little  leakage 
through  the  pile  joints,  which  were  packed  with  oakum.  .  .  . 
The  long  sides  of  the  cofferdam  are  braced  with  i2Xi2-inch 
horizontal  transverse  struts  9  feet  7  inches  apart  on  centers, 


2l8  COFFERDAMS  CHAP.  VI 

in  four  tiers  about  6  feet  apart.  At  the  rounded  ends  the  in- 
side waling  pieces  are  made  like  arch  centers  of  i2Xi2-inch 
double-scarf  pieces,  with  radial  braces  to  the  middle  of  the 
adjacent  cross-strut." 

Some  of  the  concrete  piers  for  a  bridge  ocross  the  Illinois 
River  at  Peoria,  111.,  were  founded  on  bedrock  20  feet  below  the 
bottom  of  the  river,  where  the  depth  of  water  was  approximately 
20  feet.  To  build  these  piers,  cofferdams  of  steel  sheet-piling 
on  frames  were  used.  By  means  of  an  orange-peel  bucket  the 
material  of  the  river  bottom  was  first  dredged  down  to  a  layer 
of  slate  and  soapstone,  about  3  feet  thick,  which  overlaid  the 
rock.  The  excavation  was  made  over  a  large  area  so  that  the 
material  overlying  the  slate  would  stand  at  its  natural  slope 
and  still  leave  an  area  on  the  slate  of  sufficient  size  for  the 
cofferdams,  one  of  which  was  39  by  40  feet  in  plan. 

l"The  steel- piling  forming  the  sides  and  ends  of  the  coffer- 
dam was  braced  across  the  latter  with  five  longitudinal  and 
six  transverse  rows  of  12X1 2-inch  timbers  to  hold  it  in  place 
when  the  water  had  been  drawn  down  in  the  cofferdam.  These 
timbers  were  placed  in  nine  horizontal  layers,  varying  from 
i\  to  5  feet  apart  from  the  bottom  to  the  top  of  the  cofferdam. 
The  horizontal  layers  were  held  apart  by  a  vertical  12X1 2-inch 
timber  at  each  intersection  of  the  rows  of  braces.  The  timber 
crib  formed  by  these  braces  and  verticals  was  built  in  the  water 
approximately  over  the  site.  The  horizontal  layer  which 
would  come  at  the  level  of  the  top  of  the  slate  and  soapstone 
in  the  cofferdam  was  first  assembled  as  a  raft  on  which  the 
verticals  were  erected  and  then  the  second  horizontal  layer 
was  placed,  sinking  the  crib  thus  formed  to  the  water-level. 
The  various  horizontal  layers  were  thus  added  in  succession 
and  when  they  had  been  completed  the  crib  was  towed  over  the 
site,  sunk  in  position  and  anchored." 

The  steel-piling,  of  the  Friestedt  form,  was  driven  around 
this  framework  through  the  slate  and  soapstone  to  rock,  after 
which  the  material  which  had  been  previously  dredged  was 
backfilled  around  the  cofferdam  up  to  low  water-level.  After 

1  Engineering  Record,  vol.  55,  page  247,  March  2,  1907. 


ART.  70  STEEL    SHEET-PILE    COFFERDAMS  2IQ 

pumping  out  the  cofferdam  the  layer  of  slate  and  soapstone 
was  removed  and  the  pier  built. 

Among  the  deepest  cofferdams  that  have  ever  been  placed 
are  those  used  in  founding  the  piers  of  the  Tunkhannock  Via- 
duct of  the  Delaware,  Lacka wanna  &  Western  Railroad. 
These  were  land  cofferdams  and  had  a  maximum  depth  of 
nearly  100  feet,  with  a  depth  of  65  feet  below  ground  water- 
level.  In  principle  they  closely  resemble  the  method  used  in 
placing  piers  for  buildings  as  described  in  Art.  124,  and  differ 
from  the  regular  caisson  since  excavation  took  place  simul- 
taneously with  the  driving  of  the  sheet-piling,  and  since  the 
lower  part  of  the  sheet-piling  served  as  a  form  for  the  pier 
footing. 

x"The  cofferdam  for  pier  4  is  typical  of  those  of  piers  3,  5,  6,  7 
and  8  and  was  commenced  by  assembling  on  the  surface  of  the 
ground  a  43 X 49-foot  rectangle  made  of  i2Xi2-inch  horizontal 
timbers  spliced  together  to  form  one  course  of  inner  wales. 
Vertical  posts  were  set  up  on  this  course  and  supported  a  second 
similar  course  about  16  feet  above  it,  and  two  corresponding 
courses  of  exterior  wales  were  erected  outside  of  these  and 
about  6  inches  in  the  clear  from  them." 

Lackawanna  steel  sheet-pile  units  30  feet  long  were  then 
placed  between  the  outer  and  inner  wales  and  driven  by  a 
steam-hammer  going  round  and  round  the  cofferdam  driving 
each  pile  unit  2  or  3  feet  at  a  time.  As  the  piling  was  driven  the 
interior  was  excavated  and  the  cofferdam  braced  with  succes- 
sive tiers  of  i2Xi2-inch  longitudinal  and  transverse  struts. 

After  driving  this  set  of  piling  to  its  full  length  an  exterior 
row,  concentric  with  the  inner  row  and  4  feet  8  inches  beyond 
the  same,  was  assembled  and  first  driven  to  a  penetration  of 
about  12  to  15  feet.  The  space  between  the  two  rows  was 
then  excavated  and  at  the  same  time  the  inner  row  was  also 
driven,  the  upper  tiers  of  bracing  of  the  latter  being  transferred 
to  the  bottom  and  new  sets  of  bracing  furnished  to  the  outer 
piling.  In  this  way,  by  driving  both  outer  and  inner  rows  to 
their  required  positions,  the  excavation  was  carried  to  rock. 

1  Engineering  Record,  vol.  67,  page  485,  May  3,  1913. 


22O 


COFFERDAMS 


CHAP.  VI 


The  advantage  of  two  rows  of  piling  was  in  the  easier  driving 
thereby  obtained.  The  lower  part  of  the  excavation  was  com- 
pletely filled  with  concrete,  the  steel-piling  serving  as  a  form; 


- --0,91- 


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the  surface  of  the  piling  was  protected  from  the  concrete  by 

tarred  paper,  thus  permitting  the  piling  to  be  with-drawn  later. 

Fig.  70^  illustrates  a  somewhat  similar  type  of  cofferdam  used 


ART.    71    SELF-SUPPORTING    STEEL    SHEET-PILE    COFFERDAMS     221 

in  the  reconstruction  of  the  Union  Pacific  Railroad  bridge  at. 
Kansas  City.  The  upper  tier  of  sheet-piling  was  of  wood.  The 
details  of  the  bracing  are  clearly  shown  in  the  illustration. 

Fig.  joe  is  a  half-tone  showing  the  details  of  the  bracing  used 
for  the  steel  sheet-pile  cofferdam  at  the  Loomis  St.  tunnel, 
Chicago.  The  cofferdam  was  75  by  53  feet  in  plan  and  the 
maximum  head  of  water  on  it  was  about  53  feet.  The  bracing 
consisted  of  i2Xi2-inch  timbers,  spaced  8  feet  apart  hori- 
zontally and  4  feet  vertically. 

Few  examples  exist  of  the  type  of  cofferdam  consisting  of 
steel  sheet-piling  on  cribs.  The  reason  for  this  lies  in  the  fact 
that  almost  all  the  crib  and  sheet-pile  cofferdams  have  been 
built  in  localities  where  timber  is  abundant  and  for  this  reason 
sheet-piling  of  wood  is  cheaper  than  that  of  steel. 

ART.  71.     SELF-SUPPORTING  STEEL  'SHEET-PILE   COFFERDAMS 

There  is  a  type  of  cofferdam  using  steel  sheet-piling  which 
has  almost  no  parallel  in  the  wooden  sheet-pile  cofferdam;  this 
is  the  cofferdam  without  horizontal  guides  or  bracing.  Two 
reasons  may  be  given  for  this  fact:  First,  with  the  positive 
form  of  interlock  which  most  forms  of  steel  sheet-piling  po- 
sess,  sufficient  guidance  is  furnished  by  the  interlock  to  do 
away  with  the  necessity  of  horizontal  guides;  and  second, 
the  higher  strength  lessens  the  amount  of  bracing  necessary. 

The  two  most  notable  examples  of  this  type  of  cofferdam 
are  those  used  for  the  United  States  Government  lock  at  Black 
Rock  Harbor,  Buffalo,  N.  Y.,  and  for  raising  the  United  States 
Battleship  " Maine"  in  Havana  Harbor,  Cuba.  Both  of  these 
structures  rank  high  as  daring  pieces  of  cofferdam  work,  the 
former  on  account  of  its  great  size  and  the  latter  because  of 
its  great  height. 

The  Black  Rock  cofferdam  was  built  to  permit  the  construc- 
tion of  a  ship  lock,  and  was  rectangular  in  plan,  260  by  947 
feet  over  all  as  shown  in  Fig.  jia.  The  depth  of  water  at  the 
site  varied  from  2  to  15  feet,  averaging  about  8  feet,  while  the  solid 
rock  on  which  the  lock  was  built  was  about  40  feet  below  mean 
water-level.  As  shown  in  Fig.  71^  the  sides  of  the  cofferdam 


222 


COFFERDAMS 


CHAP.  VI 


were  made  of  two  walls  of  steel  sheet-piling,  the  space  between 
the  two  walls  being  divided  into  pockets  30  feet  square  by 
transverse  walls  of  the  same  piling  as  that  used  for  the  main 
walls,  which  served  to  connect  the  latter.  A  horizontal  15- 
inch,  40-pound  channel  was  bolted  to  the  tops  of  the  piles  of  the 
inner  wall  and  a  similar  channel  was  bolted  at  an  inclination 
across  the  transverse  walls  as  shown  in  Fig.  yic. 

The  piling  was  driven  to  rock  and  at  first  wooden  guide 
piles  and  wales  were  used  to  maintain  the  alignment  of  the  steel 
sheeting,  but  eventually  these  guides  were  dispensed  with,  the 


SQUAW  ISLAND 


FIG.  7 1  a. — Plan  of  Black  Rock  Cofferdam. 

only  ones  used  being  ioX3o-foot  floating  forms  having  one 
edge  in  the  plane  of  the  sheeting.  The  fine  alignment  at- 
tained by  this  simple  method  may  be  seen  in  Fig.  71^.  After 
driving  the  piling  the  pockets  were  filled  with  clay  and  to 
further  strengthen  the  structure,  as  the  inside  was  excavated, 
a  Ijank  of  earth  25  feet  high  was  maintained  on  the  inside  as 
shown  in  Fig.  yic.  But  in  spite  of  this  bank  of  earth  the 
material  in  the  pockets  caused  the  inside  wall  to  bulge  badly 
between  the  cross  walls  in  both  a  horizontal  and  vertical 
direction. 

It  is  instructive  to  observe  the  plans  of  different  pockets  of 
the  cofferdam,  and  the  curvature  of  vertical  sections  after 
the  steel  sheet-piling  adjusted  itself  to  the  pressure  of  the  clay 
filling  by  developing  tension  in  the  interlock.  Fig.  7 1 d  gives  the 


to 


ART.  71     SELF-SUPPORTING    STEEL   SHEET-PILE   COFFERDAMS    223 


results  of  a  careful  survey  of  pocket  No.  35  in  which  the  maxi- 
mum bulging  of  sides  occurred.  It  should  be  noted  how  short  a 
distance  the  bulging  extended  below  the  sand  and  gravel  bank 
which  was  allowed  to  remain  inside  of  the  cofferdam.  The 
diagram  also  shows  vertical  sections  at  the  middle  of  pock- 
ets Nos.  30,  52,  and  75,  the  relative  location  of  the  pockets 
being  indicated  in  Fig.  710.  See  also  the  half-tone  view, 
Fig.  7  ic. 


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Natural  Bottom 


',  Gravel,  etc. 
Pocket  No.  35. 


Natural 
Bottom' 


/   •  \'W7^~\^l'^~v^a""il^\W 

Section  A-B. 
FIG.  7  id. — Diagram  Showing  Deformation  of  Steel  Sheet  Piling. 

In  all  6589  tons  of  steel  sheet-piling  were  used  in  this  coffer- 
dam, there  being  6870  linear  feet  of  piling  wall  from  45  to  50 
feet  high,  which  makes  this  the  largest  piece  of  cofferdam  sheet- 
piling  work  on  record  to  1914.  The  price  paid  the  contractor  for 
building  the  cofferdam,  which  included  furnishing  all  material, 
pumping  out,  and  maintaining  the  same,  was  $408  830.  The 
type  of  piling  used  was  that  known  as  the  Lackawanna,  and 
which  had  a  web  thickness  of  \  inch  and  weighed  40  pounds 
per  linear  foot. 


224  COFFERDAMS  CHAP.  VI 

The  cofferdam  for  raising  the  " Maine"  represents  a  special 
type  of  steel  cofferdam,  very  large  and  strong.  l  "The  problem 
was  to  surround  the  wreck  of  the  vessel,  lying  in  about  29  to  37 
feet  of  water,  with  a  cofferdam,  which  when  unwatered  would 
be  tight  enough  to  prevent  leakage,  strong  enough  to  resist 
outside  water  and  mud  pressures,  and  a  protection  that  would 
assure  safety  during  the  work.  The  cofferdam  should  be  self- 
sustaining,  if  possible.  Bracing  by  struts  across  its  interior  to 
resist  the  water  and  mud  pressures  might  be  difficult  to  install 
and  would  interfere  with  the  operation  of  removal.  The  bor- 
ings indicated  bad  conditions  for  foundations.  The  building 
of  a  cofferdam  without  internal  bracing,  which  would  withstand 
pressures  from  a  head  of  37  feet  of  water  and  practically  21  to 
23  feet  of  mud,  was  an  unprecedented  task. 

"The  cofferdam  should  be  not  only  self-sustaining  and  safe 
against  the  pressures  to  which  it  was  to  be  exposed,  but  it  should 
also  be  capable  of  complete  removal  after  it  had  served  its 
purpose.  It  should  be  able  to  support  more  or  less  superim- 
posed loads,  for  working  platforms  had  to  be  built  upon  it.  The 
work  of  unwatering  the  area  enclosed  had  to  be  carried  on  from 
the  top  of  the  cofferdam;  and  afterward,  men  and  materials  had 
to  be  transferred  from  there  to  the  interior,  for  work  upon  the 
wreck.  .  .  .  The  cofferdam  decided  upon  consisted  of  20 
equal  cylinders,  50  feet  in  diameter,  and  composed  of  steel- 
piling  75  feet  long.  ..."  A  plan  is  shown  in  Fig.  jie. 

"The  length  of  the  major  axis  of  the  cofferdam  was  prac- 
tically 399  feet,  and  of  the  minor  axis  219  feet,  leaving  a 
2o-foot  clearance  at  the  submerged  bow  of  the  ship  and  a  i4-foot 
clearance  at  the  stern,  with  45  feet  at  the  side  cylinders.  Such 
clearance  was  necessary  to  avoid  portions  of  the  wreck  which 
had  been  blown  beyond  the  position  occupied  by  the  hull. 

"The  units  of  the  cofferdam  were  made  cylindrical  for  the 
reason  that  the  extremely  high  pressures,which  would  be  exerted 
by  the  mud  rilling,  would  act  radially  and  uniformly  on  each 
pile,  straining  each  joint  to  the  same  amount  at  equal  depths, 

1  Bulletin  No.  102,  Lackawanna  Steel  Co.,  Buffalo,  N.Y. 


ART.  71    SELF-SUPPORTING    STEEL    SHEET-PILE    COFFERDAMS     225 


Q  O 
-+  -h 


FIG.   jie. — Plan  of  Cofferdam  for  Raising  the  "Maine." 


Is 

to  O 


FIG.   7 1/. — Connection  of  Cofferdam  Cylinders. 


FIG.   71^. — Filling  Clay  into  Cylinder  A.     Part  of  B  in  Foreground. 


226  COFFERDAMS  CHAP.  VI 

and  in  the  entire  cofferdam  cylinders  would  deform  least  from 
play  in  the  piling  interlocks." 

The  cylinders  were  driven  tangent  to  one  another  and  to  in- 
sure their  stability  and  prevent  leakage  of  water  through  them 
when  the  cofferdam  was  pumped  out  they  were  filled  to  the  top 
with  clayey  material  that  was  dredged  from  the  bottom  of  the 
harbor.  A  curved  diaphragm  of  steel-piling,  as  shown  in  Fig. 
7 1/,  was  driven  to  connect  adjacent  cylinders,  and  the  space 
between  this  arc  and  the  outer  surfaces  of  the  large  cylinders 
was  likewise  filled  with  dredged  material. 

The  piling  used  was  the  Lacka wanna  section,  weighing  35 
pounds  per  linear  foot,  and  had  a  web  f  inch  thick.  The  piles 
were  driven  so  that  their  tops  were  2  or  3  feet  above  normal 
water-level  (Fig.  yig)  and  the  75-foot  length  of  piling,  which 
penetrated  the  harbor  bottom  to  a  distance  of  approximately  35 
feet,  was  made  of  two  lengths  spliced  together  with  channels. 

ART.  72.     CRIB  COFFERDAMS 

Where  the  cofferdam  is  to  rest  on  bedrock  which  is  ap- 
proximately smooth  and  level,  a  crib  cofferdam,  formed  with  one 
or  two  walls  of  squared  horizontal  timbers  laid  closely,  may  be 
used  in  place  of  the  sheet-pile  cofferdam.  Where  the  single- 
wall  type  is  used  it  is  ordinarily  made  an  integral  and  permanent 
part  of  the  pier,  and  as  such  is  not  a  cofferdam,  but  a  caisson. 
For  a  description  of  this  type  see  Art.  83. 

In  his  book  on  Sub-aqueous  Foundations,  FOWLER  describes  a 
double- wall  crib  cofferdam  used  by  the  C.  B.  &  Q.  R.  R., 
which  was  made  from  2 X 8-inch  and  2Xio-inch  fence  boards 
laid  flat.  The  two  walls  were  thoroughly  tied  together  and  the 
space  between  filled  with  puddle. 

Fig.  72  a  shows  a  polygonal  cofferdam  of  the  crib  type  which 
was  used  for  the  center  pier  of  the  Arthur  Kill  Bridge.  At  the 
site  of  the  cofferdam  the  depth  of  water  at  high  tide  was  about 
28  feet,  with  about  4  feet  of  mud  and  clay  overlying  bedrock. 
This  mud  and  clay  was  dredged  out  previously  to  placing  the 
cofferdam.  The  latter  had  twelve  sides  with  walls  4  feet  apart 
in  the  clear,  and  in  this  space  puddle  was  dumped.  All  courses 


ART.  72 


CRIB    COFFERDAMS 


227 


of  timber  were  thoroughly  drift-bolted  together  and  all  joints 
caulked  with  cotton  wi eking.  No  internal  bracing  was  used. 
Before  pumping  out  the  water  a  4-foot  layer  of  concrete  was 
deposited  all  over  the  bottom  and  allowed  to  harden  for  a  week. 
The  cofferdam  for  the  new  inlet  tower  of  the  St.  Louis  Water- 
works was  of  the  double- wall  crib  type,  38  by  76  feet  in  plan 
and  22  feet  high.  The  walls  were  composed  of  horizontal 
12 X i2-inch  material  and  were  3  feet  apart  in  the  clear.  The 
joints  between  all  courses  were  carefully  caulked.  The 
cofferdam  was  braced  transversely  by  three  vertical  rows  of 


Inside  Radius  22 

Outside     »      28' 


nearly   Level. 
Section  of  Dam. 


Plan  of  Dam. 
FIG.   72a. — Cofferdam  for  Pivot  Pier  of  Arthur  Kill  Bridge. 

horizontal  i2Xi2-inch  timbers  spaced  4  feet  apart  vertically, 
and  extending  from  outside  wall  to  outside  wall,  thus  tying  the 
walls  together  as  well  as  bracing  the  cofferdam.  The  ends  were 
braced  by  similar  horizontal  i2Xi2-inch  diagonal  timbers, 
running  at  an  angle  of  about  45  degrees  from  the  center  of 
the  ends  to  the  sides. 

The  river  bottom  was  bedrock  and  the  depth  of  water  about 
15  feet,  the  current  having  a  velocity  of  from  6  to  8  miles  an 
hour.  The  cofferdam  was  held  in  place  by  three  triangular 


228  COFFERDAMS  CHAP.  VI 

cribs  filled  with  rocks  and  sunk  upstream  from  the  cofferdam 
and  tied  to  the  latter  by  cables.  The  puddle  chamber  was 
partly  filled  with  concrete  in  sacks  and  puddle  placed  on  top. 
Sacks  oLclay  were  also  banked  around  the  outside. 

Cofferdams  are  widely  used  as  temporary  adjuncts  to  open 
and  pneumatic  caissons,  but  as  the  details  differ  widely  from 
the  types  described  in  this  chapter  and  resemble  closely  the 
caissons  themselves  they  will  be  described  in  the  chapters 
dealing  with  such  caissons. 

ART.  73.     MOVABLE  COFFERDAMS 

Unless  it  forms  an  obstruction  to  navigation  only  that  part 
of  the  cofferdam  above  low  water  is  sometimes  removed. 
This  is  because  the  salvage  value  of  the  material  is  less 
than  the  cost  of  getting  it  out,  except  where  steel  sheet-pil- 
ing is  used. 

Where  the  same  size  and  style  of  cofferdam  is  to  be  used 
for  a  number  of  piers  it  will  often  prove  advantageous  to  so 
construct  a  cofferdam  that  it  can  be  used  over  and  over  again. 
In  one  type,  that  of  the  cofferdam  on  grillage,  it  is  so  easy  to 
make  its  sides  removable  that  it  is  universally  done,  even 
though  they  may  not  be  used  a  second  time. 

A  movable  cofferdam  consisting  of  sheet-piling  supported 
by  a  crib  was  used  in  constructing  the  piers  of  the  Falls-of- 
Schuylkill  Bridge,  of  the  Philadelphia  &  Reading  Railroad. 
When  in  position  the  cofferdam  was  62  feet  long,  36  feet  wide 
and  1 6  feet  high.  The  cribs  were  10  feet  thick,  making  the 
inside  dimensions  42X16  feet  The  cofferdam  was  divided 
vertically  through  each  short  side  into  two  parts  of  equal  size 
and  these  were  floated  separately  to  the  site,  joined  together 
and  sunk.  Each  section  had  water-tight  compartments  to  assist 
in  floating  and  these  were  filled  with  water  and  stone,  while 
other  non-water-tight  compartments  were  filled  with  stone, 
when  it  was  desired  to  sink  the  sections.  On  reaching  the 
rock  bottom  sheet-piling  of  jointed  planks,  3  or  4  inches  thick, 
was  placed  on  the  outside  and  spiked  there.  Puddle  was  then 
placed  around  the  outside,  after  which  the  cofferdam  was 


ART.  73 


MOVABLE    COFFERDAMS 


229 


pumped  out.     Two  sets  of  horizontal  bracing  connecting  the 
long  sides  were  placed  as  the  water  was  removed. 

In  placing  cylinder  piers  for  the  Queen's  Bridge,  Melbourne, 
Australia,  square  movable  cofferdams  of  the  sheet-pile-on-frame 
type  were  used.  One  side  opened  outward  as  a  door,  thus  per- 
mitting the  cofferdam  to  be  removed  on  completion  of  a  pier. 
luThe  dam  was  built  on  shore  complete,  and  launched  ready 
for  immediate  use  on  the  site  of  a  cylinder.  The  sheet-piling 
was  vertical  and  consisted  of  12  X  4-inch  rough-sawn  Oregon 
planks,  supported  by  horizontal  frames  of  i2Xi2-inch  Oregon 
timber,  spaced  close  together  near  the  bottom  of  the  river,  to 


J"x<?' 


6"*8" 


25'  0" 


6'xlO* 


Longitudinal  Section. 


Cross  Section. 


•3'xfT 

BothSidei 

Smoothed 

andEdges 

Beveled 


Plan-  Corner  Details. 

FIG.  730. — Cofferdam  Used  on  Key  West  Extension  of  Florida  East  Coast  Railway. 

carry  the  greater  pressure  of  water.  Up  the  four  corners  of  the 
dam  were  i2Xi2-inch  Oregon  timbers,  into  which  the  frames 
were  checked  and  by  which  they  were  kept  to  their  proper 
spacing,  and  which  formed  supports  for  the  door.  Outside 
the  sheet-piling,  at  the  top  and  bottom  frames  there  were 
outside  wales,  12  by  6  inches  (keeping  the  sheet-piling  in  place), 
bolted  to  the  frames  inside  by  i-inch  bolts,  two  to  each  wale, 
passing  between  two  sheet  piles."  The  sheet-piling  was  flush 

1  Engineering  News,  vol.  33,  page  230,  April  4,  1895. 


230 


COFFERDAMS 


CHAP.  VI 


with  the  bottom  of  the  frame  and  extended  a  few  feet  above 
the  top. 

At  the  site  of  the  piers  there  was  about  3  feet  of  soft  silt 
covering  the  rock.  This  silt  was  covered  with  puddle  before 
placing  the  cofferdam.  After  sinking  it  by  weighting,  the  sheet- 
piling  was  driven  through  the  puddle  and  silt.  On  pumping 
out  the  cofferdam  much  of  the  silt  ran  into  the  interior  and  the 
clay  took  its  place,  thus  sealing  the  structure.  To  remove  the 
cofferdam  the  sheet-piling  was  first  drawn  up,  the  loading 
taken  off,  the  door  opened,  and  the  cofferdam  floated  out.  At 


Base  of  Rail,  El.  +11.05 


\Ffemeve  this  Top  Cross  -Bruce  and 
-,..\brace  Concrete  Form  from  Side 
•  \Pcsfe  as  indicated  in  dotted  Line 
j  \pefore  Coping  -Course  is  put  on  f 


ffxff" 


•Hh 


lajWA 


Loncfitu 


12x12 


5,11 


\ 


Ih7l  I   II   II   II   II 

Timbers 


Part  Sectional    Side    Elevation.  Sectional    Side    Eevaiion. 

FIG.   73&.  —  Cofferdam  for  Rest  Pier  of  Chicago  and  Northwestern  Railway  Lift 

Bridge  at  Kinzie  Street,  Chicago. 

first  tarpaulin  was  placed  around  the  outside  of  the  cofferdam 
but  it  was  later  found  that  this  was  unnecessary  since  the 
sheet-piling  was  water-tight  without  it. 

Fig.  73  a  illustrates  the  form  of  a  movable  cofferdam  used  in 
constructing  the  piers  of  the  Key  West  Extension  of  the  Florida 
East  Coast  Railway,  where  the  depth  of  water  did  not  exceed 
8  feet.  The  two  sides  and  the  two  ends  formed  independent 
portable  sections  which  were  connected  together  by  means  of 
ij-inch  vertical  rods  running  down  through  the  overlapping 
rangers  at  the  corners  of  the  cofferdam. 


ART.  73 


MOVABLE   COFFERDAMS 


231 


Detail  at  A 
4xd'about  6'0'c.Joc.  - 


At  the  site  of  the  piers  sand  overlaid  the  coral  rock.  Piles, 
for  the  foundation  of  the  pier,  were  first  driven  until  the  tops 
were  2  feet  below  low  water,  after  which  the  cofferdam  was 
assembled  on  a  barge,  lifted  from  the  same  and  set  in  place. 
The  sand  was  then  pumped  out  by  a  centrifugal  pump,  after 
which  a  2-foot  seal  of  concrete  was  placed  over  the  whole 
bottom.  After  allowing  this  concrete  to  harden  for  seven 
days,  the  cofferdam  was  pumped  out,  forms  placed  and  the 
pier  built.  On  completion  of  the 
pier  the  rods  were  withdrawn,  which 
allowed  the  sections  to  float,  free. 

MOVABLE  COFFERDAMS  ON  GRIL- 
LAGE.— On  account  of  its  conven- 
ience and  ease  of  manipulation  a 
movable  cofferdam  is  almost  uni- 
versally employed  where  a  timber 
grillage  foundation  on  piles  is  used 
for  the  pier.  The  grillage  and  the 
cofferdam  form  an  open  box  con- 
structed on  shore  or  on  a  barge  or 
raft,  launched,  floated  to  the  site, 
and  sunk  on  the  pile  foundation  by 
building  the  pier  in  the  box.  This 
type  differs  from  the  box  caisson, 
described  in  Art.  80,  since  the  sides 
of  the  former  are  not  a  permanent 
part  of  the  pier.  After  the  pier  is  built  to  above  high-water 
level  the  cofferdam  is  removed,  the  sides  being  so  fastened  to 
the  grillage  that  this  can  easily  be  done. 

Fig.  736  shows  the  details  of  the  movable  cofferdam  used  for 
the  i2X4i2^foot  pier  of  the  Kinzie  St.  drawbridge,  in  Chicago. 
This  cofferdam  was  connected  to  the  grillage  by  28  vertical 
i-inch  rods,  21  \  feet  long.  To  sink  the  structure,  concrete 
forming  the  pier  was  placed  in  the  same,  the  cofferdam  itself 
serving  as  a  form  for  the  concrete  up  to  an  elevation  shown  in 
the  drawing,  and  above  this  regular  forms  were  used.  On  com- 


JUUUUL 

Section  of  Caisson 


FIG.  73C. — Cofferdam  with 
Removable  Sides. 


232  COFFERDAMS  CHAP.  VI 

pletion  of  the  pier  the  rods  were  removed,  which  permitted  the 
removal  of  the  cofferdam  from  the  grillage. 

A  very  simple  cofferdam  on  grillage  was  used  in  building  the 
foundation  piers  of  the  Bellevue  Hospital  Boiler  House,  a 
section  of  which  may  be  seen  in  Fig.  73^.  The  largest  was 
approximately  14X52  feet  in  plan  and  12^  feet  high.  The 
most  interesting  feature  is  the  very  thin  grillage  used,  it 
being  composed  of  two  crossed  courses  of  2-inch  tongue-and- 
grooved  planks.  It  was  desired  to  use  a  thickness  which  would 
give  enough  strength  for  launching  and  sinking  stresses,  and 
yet  be  sufficiently  flexible  so  that  a  uniform  bearing  over  the 
slightly  irregular  pile  tops  would  be  secured. 

ART.  74.     MISCELLANEOUS  TYPES 

The  foregoing  articles  have  dealt  with  what  may  be  called 
standard  types  of  cofferdams,  but  there  have  been  many  coffer- 
dams constructed  which  are  either  a  combination  of  any  two 
standard  types  or  which  differ  fundamentally  from  those  which 
have  been  described.  For  instance  in  the  cofferdam  for  the 
Dearborn  St.  bridge,  Chicago,  a  double-wall  sheet-pile  coffer- 
dam was  used,  the  outer  wall  being  composed  of  Wakefield 
sheet-piling  and  the  inner  wall  of  Friestedt  steel  sheet-piling, 
thus  giving  a  composite  wood  and  steel  sheet-pile  cofferdam. 

Fig.  74<z  shows  a  form  of  cofferdam  known  as  the  A-frame 
type  which  is  used  on  bedrock.  This  particular  one  was  used 
on  the  New  York  Barge  Canal  and  consisted  of  a  series  of  bents, 
spaced  6  feet  center  to  center.  On  these  bents  rested  purlins, 
which  in  turn  supported  the  sheathing  of  jointed  and  caulked 
3-inch  planking.  As  shown  in  the  illustration,  the  structure 
was  braced  against  sliding  by  having  certain  of  the  struts 
bear  against  concrete  footings  on  the  rock.  This  form  of  bear- 
ing can  be  made  only  when  the  rock  is  exposed  at  times.  The 
maximum  head  of  water  supported  was  18  feet. 

Another  cofferdam  of  the  same  type  was  used  for  some  canal 
work  at  Keokuk,  Iowa.  Here  it  was  necessary  to  construct 
the  cofferdam  without  drawing  off  the  water  in  the  canal, 


ART.  74 


MISCELLANEOUS   TYPES 


233 


and  hence  it  was  built  away  from  the  site,  brought  to  place, 
and  sunk  by  weighting  with  iron  rails.  Water- tightness  was 
promoted  by  covering  the  sheathing  with  canvas. 

In  constructing  concrete  wharves  at  Fort  Mason,  San  Fran- 
cisco, steel-cylinder  cofferdams,  7  feet  7  inches  in  diameter  and 
50  feet  long,  were  used  in  which  to  construct  reinforced-con- 
crete  piers.  These  cylinders,  weighing  17  tons  each,  were 
driven  by  a  pile-driver  through  the  bottom  and  into  hard-pan, 
after  which  the  water  was  bailed  out,  the  mud  removed,  wooden 


™---/O    — •*  /          ^-£O — - 

Concrete  Footings  x» — 
B  "wide  at  each  Dent     " 


FIG.   740. — A-Frame  Cofferdam  Used  on  New  York  State  Barge  Canal. 

forms  placed  and  the  4-foot  reinforced-concrete  piers  with  en- 
larged bases,  6^  feet  in  diameter,  cast.  After  the  concrete  had 
set  the  cylinders  were  pulled  by  the  pile-driver,  the  required 
pall  being  about  50  tons. 

In  vol.  26  of  Revue  Technique,  the  proposed  design  for  a 
cofferdam  of  ice  to  close  the  entrance  to  the  outer  basin  at  the 
Port  of  La  Rochelle  is  described.  The  freezing  was  to  be  done 
with  refrigerating  machines,  sheathings  of  non-conducting 
material  being  placed  between  the  water  to  be  frozen  and  that 


234  COFFERDAMS  CHAP.  VI 

to  be  left  unfrozen.     It  was  estimated  that  such  a  cofferdam 
would  cost  less  than  any  ordinary  form. 

Another  proposed  type  described  in  the  same  article  was  of 
reinforced  concrete.  It  was  designed  with  inclined  sides,  the 
width  of  the  base  being  19.7  feet,  the  width  of  the  top  4.9  feet, 
and  the  thickness  of  the  walls  about  10  inches.  The  walls  were 
to  be  well  braced  by  struts.  The  structure  was  to  be  built 
away  from  the  site,  floated  to  place  and  sunk  by  filling  with 
clay.  It  was  estimated  that  it  would  have  sufficient  stability 
to  withstand  a  head  of  24.6  feet  of  water. 

ART.  75.     PUDDLE  AND  LEAKAGE 

It  is  seldom  attempted  to  construct  a  water-tight  cofferdam, 
for  the  cost  of  such  is  usually  prohibitive.  The  greatest 
trouble  from  leakage  occurs  in  the  case  of  cofferdams  which 
rest  on  rock,  for  here  it  is  almost  impossible  to  prevent  the 
water  from  running  in  between  the  bottom  of  the  cofferdam  and 
the  rock.  But  if  the  structure  rests  on  clay  and  the  sheet- 
piling  is  driven  well  down  there  will  be  but  a  slight  amount  of 
leakage  at  this  point.  Where  the  leakage  occurs  through  seams 
in  the  rock  it  may  be  stopped  by  filling  the  seams  with  grout 
pumped  in  through  pipes  about  4  inches  in  diameter.  Another 
method  is  to  dump  clay,  sand,  ashes,  etc.,  all  around  the  coffer- 
dam with  a  view  of  shutting  off  the  water-supply  of  the  crevices. 
When  the  leakage  is  due  to  irregularities  in  the  rock  surface, 
concrete  in  bags  may  be  placed  on  the  bottom,  or  water-logged 
oat  straw  may  be  sunk  by  mixing  with  ashes  or  covered  with 
a  wire  net  loaded  with  sand  and  clay,  after  which  the  rest 
of  the  puddle  filling  may  be  placed.  Another  method  of  pre- 
venting leakage  on  a  rock  bottom  is  to  use  canvas  as  noted  in 
previous  articles. 

Leaks  often  develop  in  double-wall  cofferdams  by  the  filling 
between  the  walls  not  compacting  well,  or  settling  after  being 
placed  and  leaving  openings  beneath  cross  braces.  To  com- 
pact this  filling  piles  are  sometimes  driven  into  it  or  stock  ram- 
ming may  be  resorted  to.  The  latter  consists  of  forcing 
clay  cylinders  through  pipes  into  the  filling. 


ART.  76  DESIGN   OF   COFFERDAMS  235 

The  ideal  puddle  for  a  cofferdam  is  a  mixture  of  clay  and  sand 
or  clay  and  gravel.  The  sand  or  clay  alone  is  almost  worth- 
less, the  sand  because  of  its  permeability  and  the  clay  on  account 
of  its  tendency  to  allow  a  leak  once  started  to  enlarge  rapidly 
through  the  clay  arching  over  the  leak,  instead  of  falling  into  and 
stopping  the  same.  The  ideal  mixture  obtains  when  there  is 
just  enough  coarse  material  in  it  to  reduce  the  cohesion  of  the 
mass  sufficiently  to  prevent  this  arching  action. 

ART.  76.     DESIGN  OF  COFFERDAMS 

Like  most  structures  used  in  foundations,  a  purely  theoretical 
design  of  cofferdams  leads  to  unsatisfactory  results.  For  some 
types  it  is  a  simple  matter  to  design  the  structure  to  resist  the 
hydrostatic  pressure,  but  to  design  it  properly  to  resist  safely  the 
pressure  of  the  earth  filling,  or  of  freshets,  ice,  or  floating  logs, 
requires  much  experience. 

Earth  cofferdams  usually  fail  by  the  water  seeping  through 
and  enlarging  a  channel  until  a  washout  takes  place.  For  this 
reason  such  cofferdams  should  be  carefully  watched  to  detect 
small  leaks  that  they  may  be  checked  quickly  after  starting. 
In  general,  if  the  cofferdam  is  madeo  f  a  good  mixture  of  clay  and 
sand,  has  a  width  of  at  least  3  feet  at  the  top,  which  is  well  above 
high  water,  and  has  sides  inclined  at  the  natural  slope  of  the 
material,  the  cofferdam  will  be  safe. 

In  the  single-wall  sheet-pile  cofferdam  with  guide  piles,  if  the 
wales  are  at  the  top  and  bottom,  the  sheet  piles  may  be  assumed 
to  act  as  simple  beams,  with  a  load  per  vertical  foot  varying 
uniformly  from  zero  at  the  water  surface  to  a  value  of  wd 
pounds  per  square  foot  at  the  surface  of  the  earth,  where  w  is 
the  weight  in  pounds  of  a  cubic  foot  of  water  and  d  the  depth  of 
the  water  in  feet.  For  a  discussion  of  the  design  of  sheet-piling 
see  Art.  63.  The  wales  take  the  reactions  of  the  sheet-piling 
and  transfer  them  as  supported,  partially  continuous,  or  con- 
tinuous beams,  to  the  guide  piles.  Conservative  engineers 
usually  design  the  wales  as  simple  or  supported  beams.  If  no 
bracing  is  used  the  guide  piles  should  be  designed  as  cantilever 
beams  with  loads  coming  from  the  waling  pieces.  The  maxi- 


236  COFFERDAMS  CHAP.  VI 

mum  moment  will  occur  at  or  below  the  mud  line.  If  firmly 
braced  at  the  top  by  struts  extending  across  the  cofferdam,  it 
will  be  best  to  design  the  guide  piles  as  simple  beams. 

Each  wall  of  the  double-wall  sheet-pile  cofferdam  with  guide 
piles  may  be  designed  somewhat  in  accordance  with  the  above 
outline.  The  outer  wall  will  be  subjected  to  water  pressure 
from  the  outside  and  earth  pressure  from  the  inside.  Expe- 
rience shows  that  usually  the  pressure  from  the  puddle  will,  for 
equal  heads,  be  larger  than  the  pressure  from  the  water.  This 
will  cause  a  stress  in  the  tie  rods  connecting  the  two  walls. 
The  inner  wall  must  be  designed  to  resist  the  forces  due  to  the 
puddle  filling. 

In,  the  design  of  a  cofferdam  composed  of  sheet-piling  on 
frames  and  the  corresponding  bracing,  the  hydrostatic  pressure 
is  the  only  force  to  be  considered.  The  sheet-piling  acts  as  a 
simple  beam  between  horizontal  rangers,  and  the  latter  act  as 
beams  between  bracing  struts. 

The  sheet-pile-on-crib  cofferdam  must  be  designed  so  that 
the  cribs  will  not  overturn  or  slide.  To  be  safe  against  sliding 
the  weight  of  the  cribs  and  filling  per  linear  foot  of  length, 
multiplied  by  the  coefficient  of  friction  between  the  crib  and 
rock,  must  be  greater  than  wd2/2,  where  the  terms  have  the  same 
meaning  as  those  previously  given.  To  be  safe  against  over- 
turning the  weight  per  linear  foot  of  length,  including  filling, 
multiplied  by  one-half  the  width,  must  be  greater  than  wd*/6. 

Before  attempting  to  design  a  cofferdam  the  literature  on  the 
subject  should  be  carefully  read,  for  as  stated  in  the  first  part  of 
the  article  no  purely  theoretical  design  will  result  in  a  thoroughly 
satisfactory  structure.  In  the  preceding  articles,  standard 
types  and  standard  methods  of  construction  are  described  and 
a  careful  reading  of  this  material  will  help  the  inexperienced 
engineer.  For  more  detailed  information  the  reader  is  referred 
to  the  carefully  selected  list  of  references  in  Chap.  XIX. 

ART.  77.     COST  OF  COFFERDAMS 

Little  value  attends  the  mere  statement  of  cost  of  engineering 
works  unless  all  the  conditions  are  fully  described  (see  Engineer- 


ART.  77  COST    OF    COFFERDAMS  237 

ing  NeHvs,  vol.  70,  page  1305,  Dec.  25,  1913).  For  this  reason 
only  a  few  figures  will  be  given  here.  In  Art.  71  the  cost  to 
the  United  States  of  the  steel  sheet-pile  cofferdam  at  Black 
Rock  Harbor  is  given. 

THOMAS  P.  ROBERTS  writes1  that  for  large  cofferdams  con- 
structed in  the  rivers  near  Pittsburgh,  Pa.,  the  cost  per  linear 
foot  of  cofferdam  will  vary  from  $8  to  $10.  These  cofferdams 
are  of  the  double-wall  type,  from  10  to  12  feet  wide  and  from  14 
to  1 6  feet  high.  Two-inch  hemlock  sheet-piling  is  used. 

In  constructing  some  piers  for  a  bridge  over  Paint  Creek, 
near  Chillicothe,  Ohio,  where  the  water  was  from  3  to  6  feet 
deep,  a  single-wall  steel  sheet-pile  cofferdam,  16  by  62  feet  in 
plan,  was  used.  The  piling  was  16  feet  long  and  was  driven 
into  the  gravel  bottom  until  the  top  of  the  same  was  2  feet 
above  water-level.  The  bracing  consisted  of  two  horizontal 
wales  at  the  top,  running  longitudinally  and  cross-braced  with 
struts.  The  first  cost  of  the  sheet-piling  was  about  $1822,  and 
as  the  same  piling  was  used  for  five  cofferdams,  the  cost  per 
cofferdam  was  about  $364.  The  cost  of  placing  two  of  the  coffer- 
dams averaged  $94,  while  the  cost  of  removing  the  piling  per 
cofferdam  was  $47,  thus  making  the  total  cost  of  each  cofferdam 
about  $505. 

In  1886,  near  the  same  site,  some  cofferdams  of  the  double- 
wall  type  were  built  with  wooden  sheet-piling  on  guide  piles. 
For  the  river  piers  the  cofferdams  were  22  by  45  feet  inside  and 
35  by  58  feet  outside.  The  guide  piles  were  about  8  feet  apart. 
The  wales  were  3  inches  thick  and  the  sheet-piling  was  made  of 
2-inch  planking.  The  bid  for  the  construction  of  these  two 
cofferdams  averaged  about  $569,  the  unit  prices  being  as  fol- 
lows: timber  $24  per  1000  feet  B.  M.,  piles  30  cents  per  linear 
foot,  iron  bolts  5  cents  per  pound,  and  earth  filling  in  cofferdam 
30  cents  per  cubic  yard.  At  the  time  the  steel  cofferdams  were 
built  (about  1905)  the  cost  of  the  double- wall  cofferdams  would 
probably  have  been  between  30  and  40  percent  greater  than  in 
1886.  These  figures  show  the  considerable  economy  of  coffer- 
dams in  which  steel  instead  of  timber  sheet-piling  is  employed. 

1  Engineering  News,  vol.  54,  page  138,  Aug.  10,  1905. 


238  COFFERDAMS  CHAP.  VI 

ART.  78.     CHOICE  OF  TYPE 

The  best  type  to  use  in  any  particular  case  is  that  one  which 
fulfills  all  the  required  functions  at  a  minimum  cost.  Where  the 
depth  of  water  is  not  great,  and  the  danger  of  overflow  and  wash- 
ing away  does  not  occur,  the  simple  earth  cofferdam  will  prove 
the  cheapest  and  most  satisfactory,  especially  if  the  site  of  the 
permanent  foundation  must  be  excavated  to  some  depth;  for 
in  this  case  the  excavated  material  may  be  used  to  form  the 
cofferdam.  Where  the  depth  of  water  is  considerable  the 
width  of  the  cofferdam  becomes  so  great  that  this  type  is  not 
economical. 

Where  the  bottom  can  be  penetrated  with  piles  the  sheet-pile 
cofferdam  with  guide  piles  is  a  very  satisfactory  type.  For 
high  heads  the  double-wall  cofferdam  will  be  used.  This  form 
approximates  somewhat  the  earth  cofferdam,  but  possesses  the 
advantage  over  it  that  less  earth  is  required,  and  it  is  also  a 
stronger  structure  and  more  nearly  water-tight.  The  single- 
wall  type  obstructs  the  water-way  less  than  does  that  with 
double  walls,  but  it  has  less  strength.  If  bracing  can  be  used 
on  the  inside  the  latter  can  usually  be  made  sufficiently  strong 
to  withstand  any  forces  that  are  likely  to  come  upon  it. 

Where  the  bottom  is  composed  of  rock  a  sheet-pile  cofferdam 
on  a  frame  or  cribs  will  be  used.  Frames  are  used  where  the 
cofferdam  can  be  braced  across  by  struts,  but  where  the  struc- 
ture is  too  large  for  such  bracing,  cribs  are  necessary.  The  crib 
cofferdam  may  be  said  to  have  gone  out  of  use,  the  open  caisson 
having  taken  its  place.  Where  the  cofferdam  is  not  large  and 
the  same  size  is  to  be  used  a  number  of  times  some  form  of 
movable  structure  should  be  adopted. 

As  to  whether  wooden  or  steel-piling  should  be  used  in  any 
particular  case  becomes  simply  a  question  of  the  relative  cost 
of  the  two  types.  In  general,  the  steel-piling  will  be  used  in  and 
near  cities  or  where  the  work  is  in  close  proximity  to  a  railroad, 
while  the  wooden  sheet-piling  will  be  cheaper  near  centers  of 
timber  supplies.  Steel-piling  will  also  show  more  economy  the 
greater  the  depth  of  cofferdam. 


ART.  91  SINKING   OPEN  CAISSONS  279 

ting  edges  and  along  the  sides  the  frictional  resistance  is  con- 
siderably decreased.  Another  advantage  is  that  it  tends  to 
wash  the  material  toward  the  interior  of  the  caisson,  where  it 
can  be  picked  up  by  the  dredging  buckets,  which  have  previ- 
ously made  a  hole  in  the  center. 

It  is  possible  to  drive  caissons  only  when  they  are  small  and 
even  then  only  light  blows  with  the  hammer  may  safely  be  given. 
Pulling  the  caisson  down  may  sometimes  be  employed  to  advan- 
tage, if  it  is  possible  to  drive  piles  around  the  outside  and  attach 
tackle  to  them  and  to  the  sides  of  the  caisson. 


CHAPTER  VIII 
PNEUMATIC  CAISSONS  FOR  BRIDGES 

ART.  92.    THE  PNEUMATIC  PROCESS 

The  use  of  the  plenum  pneumatic  process  for  founding  deep 
piers  is  a  good  example  of  the  application  of  scientific  principles 
to  foundation  work.  A  pneumatic  caisson  may  be  defined  as  a 
structure,  open  at  the  bottom  and  closed  at  the  top — in  other 
words,  an  inverted  box — in  which  compressed  air  is  utilized  to 
keep  the  water  and  mud  from  coming  into  the  box,  and  which 
forms  an  integral  part  of  the  foundation. 

The  caisson,  which  is  usually  not  over  6  feet  high  in  the  work- 
ing chamber,  is  surmounted  by  a  crib  and  cofferdam,  the  former, 
with  the  exception  of  one  or  more  vertical  wells,  called  shafts, 
being  filled  with  concrete  as  the  caisson  sinks.  This  concreting, 
together  with  the  excavating  done  in  the  working  chamber, 
as  the  interior  of  the  caisson  is  called,  effects  the  sinking 
of  the  latter. 

The  working  chamber  must  be  practically  air-  and  water- 
tight, and  yet  there  must  be  an  opening  for  men  to  enter  and 
leave  the  chamber,  as  well  as  an  inlet  and  outlet  for  materials. 
These  openings  are  provided  by  vertical  shafts  and  air-locks. 
The  shafts,  which  extend  from  the  roof  of  the  caisson  to  a 
point  well  above  the  top  of  the  crib  and  the  level  of  the  water 
outside,  are  usually  of  a  circular  or  oval  section  and  from  i\  to 
4  feet  in  maximum  diameter.  In  the  shafts,  at  the  bottom, 
top,  or  between  these  two  points,  are  placed  the  air-locks,  they 
being  air-tight  chambers,  often  simply  a  part  of  the  shaft  itself, 
fitted  with  two  doors,  one  of  which  leads  to  the  working  chamber 
and  the  other  to  the  open  air. 

The  most  pronounced  advantage  of  the  pneumatic  caisson  as 
compared  with  the  open-caisson  process  lies  in  the  fact  that  the 

280 


ART.  92  THE   PNEUMATIC   PROCESS  281 

engineer  has  more  control  over  the  work,  having  a  better  oppor- 
tunity to  sink  the  caisson  vertically,  to  remove  large  boulders, 
sunken  logs,  etc.,  from  under  the  cutting  edge;  the  foundation 
bed  can  be  properly  prepared  and  personally  inspected;  and 
lastly,  the  concrete  filling  of  the  working  chamber  is  deposited  in 
air,  thus  giving  a  superior  foundation.  Another  point,  which  is 
sometimes  of  great  importance  in  placing  foundations  for  build- 
ings, is  that  the  soil  about  the  caisson  is  not  so  liable  to  be  dis- 
turbed when  the  pneumatic  process  is  used.  The  one  disadvan- 
tage of  this  process  is  that  the  men  have  to  work  under  an  air 
pressure  which  is  sufficient  to  balance  the  pressure  of  the  sur- 
rounding water  in  addition  to  atmospheric  pressure,  or  practi- 
cally the  full  hydrostatic  head  from  the  cutting  edge  to  the 
water  surface. 

For  depths  from  about  30  to  no  feet  this  type  of  caisson  is 
extensively  employed.  For  depths  less  than  30  feet  the  coffer- 
dam process  is  usually,  but  not  always,  a  more  economical 
method  of  placing  the  foundation  while  for  depths  greater  than 
about  no  feet,  corresponding  to  a  pressure  of  over  three 
atmospheres  above  the  normal,  the  open-caisson  method  must 
be  employed,  since  men  cannot  work  advantageously  under 
such  high  pressures. 

At  the  St.  Louis  Municipal  bridge  men  worked  at  a  maximum 
immersion  of  over  113  feet,  the  maximum  gage  pressure  being 
50  pounds,  which  is  probably  the  world  record  for  bridge 
caissons,  with  the  possible  exception  of  a  bridge  caisson  in 
Denmark,  where  it  is  reported  (Eng.  News,  vol.  26,  page 
467,  Nov.  14,  1891)  that  a  working  depth  of  115  feet  was 
reached.  Among  other  notable  examples  of  deep  immersions 
are  the  St.  Louis  arch  bridge  caissons,  109.7  feet;  tne  Memphis 
bridge  caissons,  106.4  feet;  the  Williamsburg  bridge  (New 
York)  caissons,  107.5;  and  the  Broadway  bridge  (Portland, 
Ore.)  caissons,  101  feet.  The  elevation  of  the  bottom  of 
the  deepest  caisson  (No.  4)  of  the  St.  Louis  Municipal  bridge 
is  2.1  feet  below  the  bottom  of  the  east  abutment  caisson,  or 
the  deepest  one  of  the  St.  Louis  arch  bridge. 
The  caisson  used  in  sinking  a  mine  shaft  near  Deerwood, 


282  PNUEMATIC   CAISSONS   FOR  BRIDGES  CHAP.  VIII 

Minn.,  was  sunk  to  a  depth  of  123  feet  below  ground- water 
level  and  130  feet  below  the  ground  surface.  The  maximum 
pressure  used  was  52  pounds  per  square  inch,  a  higher  value 
than  has  probably  ever  been  used  in  bridge  or  building  caissons. 

As  first  applied  the  pneumatic  caisson  process  was  a  very 
simple  affair,  the  caisson  consisting  of  a  cast-iron  cylinder,  called 
a  pneumatic  pile,  which  formed  both  the  working  chamber  and 
a  section  of  the  pier.  The  first  used  in  this  country  were  sunk 
in  1852  in  the  Pedee  River,  North  Carolina. 

The  St.  Louis  arch  bridge  was  the  first  in  this  country  to  be 
founded  on  large  pneumatic  caissons,  its  east  abutment  caisson, 
which  had  a  maximum  immersion  of  109  feet  8|  inches,  being 
sunk  in  1870.  The  second  bridge  in  this  country  to  be  founded 
on  large  pneumatic  caissons  was  the  great  Brooklyn  suspension 
bridge,  which,  in  its  New  York  tower  caisson,  sunk  in  1871, 
has  the  largest  pneumatic  caisson  ever  placed  for  a  bridge 
foundation.  It  was  102  by  172  feet  in  plan  and  was  sunk  to  a 
depth  of  78  feet  below  high-water  level. 

The  pneumatic  caisson  process  has  been  widely  used  in 
America  and  on  the  European  continent.  As  a  class,  English 
engineers  have  apparently  shown  some  aversion  to  it,  and  in 
many  cases,  where  it  seems  to  have  been  the  preferable  struc- 
ture on  account  of  the  presence  of  boulders  and  logs,  the  open- 
caisson  process  was  used.  American  engineers  have  developed 
the  wooden  caisson  to  a  high  state  of  perfection,  but  at  present 
(1914)  owing  to  the  high  price  of  timber  the  tendency  is 
toward  the  use  of  more  reinforced  concrete  and  less  timber. 
In  Europe  the  metallic  form  of  pneumatic  caisson  has  been 
extensively  used. 

To  give  some  indication  of  the  progress  made  in  the  science 
and  art  of  foundation  construction  it  is  interesting  to  note  that 
the  cost  per  cubic  yard  of  the  substructure  of  the  Municipal 
bridge  at  St.  Louis  is  only  29.6  percent  of  the  corresponding 
cost  of  that  of  the  St.  Louis  arch  bridge,  which  is  located  about 
a  mile  above  it,  and  50.8  percent  of  that  of  the  Memphis  bridge. 
The  substructures  of  these  three  bridges  were  completed  in 
1911,  1871  and  1891  respectively.  In  this  comparison  the 


ART.  93  CAISSON  ROOF   CONSTRUCTION  283 

approaches    are    excluded.     As    previously    noted,     the  three 
bridges  have  deep  foundations. 

The  contract  price  for  the  caisson  work  of  the  Municipal 
bridge  was  $27.00  per  cubic  yard  below  the  cutting  edge,  and 
$12.90  per  cubic  yard  from  the  cutting  edge  to  the  top  of  the 
crib. 


ART.  93.     CAISSON  ROOF  CONSTRUCTION 

TIMBER  ROOFS. — The  design  of  the  roof  has  always  been 
largely  a  question  of  judgment  as  it  is  almost  impossible  to 
analyze  the  stresses.  The  tendency  of  roof  construction  has 
been  constantly  to  decrease  the  thickness  of  the  timber  roof 
and  consequently  its  cost.  When  concrete  superseded  stone 
masonry  as  a  filling  for  the  crib  a  considerable  decrease  in 
the  thickness  of  the  roof  was  made  possible  on  account  of  the 
strength  of  the  concrete.  A  more  generous  use  of  bulkheads 
and  the  arrangement  of  the  bracing  above  and  below  the  deck 
to  act  as  trusses  also  aided  in  securing  a  thinner  roof.  At 
present  many  caissons  do  away  with  a  permanent  timber  roof 
almost  entirely  by  reinforcing  the  concrete  filling  of  the  crib. 

The  roof  is  usually  made  with  layers  of  12X1 2-inch  timbers, 
sheathed  on  the  lower  side  with  2-  or  3-inch  planks.  Sheathing 
may  also  be  used  between  the  courses  of  large  timbers.  The 
different  courses  run  in  different  directions;  if  the  roof  is  of 
a  two-course  thickness  both  courses  may  run  transversely, 
while  if  it  has  three  courses  the  lower  and  upper  courses  run 
transversely  and  the  middle  course  longitudinally.  All  caulk- 
ing of  the  air  chamber  is  done  from  the  inside  of  the  work- 
ing chamber,  against  the  air  blowing  out,  while  the  outside 
planking  is  caulked  from  the  outside,  to  prevent  the  water  from 
getting  in. 

The  roof  of  the  io2Xi72-foot  caisson  for  the  New  York 
tower  of  the  Brooklyn  bridge  was  composed  of  a  solid  mass  of 
squared  timbers,  22  feet  thick,  all  timbers  being  12X12  inches 
in  section,  and  thoroughly  drift-bolted  together.  This  is  the 
thickest  roof  that  has  ever  been  used. 


284 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  VIII 


The  roof  of  the  31X79- 
foot  rectangular  caisson  for 
the  old  piers  of  the  Baltimore 
and  Ohio  Railroad  bridge  at 
Havre  de  Grace  was  composed 
of  eight  thicknesses  of  1 2  X 1 2- 
inch  timbers,  the  courses 
alternating  in  direction,  some 
running  longitudinally,  others 
transversely,  and  still  others 
diagonally.  The  lower  sur- 
face was  sheathed  with  3X 
i2-inch  planks.  This  form 
of  roof  is  typical  of  a  num- 
ber of  caissons  built  under 
the  direction  of  WILLIAM 
PATTON,  who  was  an  extrem- 
ist in  respect  to  thick  roofs. 

The  roof  of  the  east  abut- 
ment caisson  of  the  St.  Louis 
arch  bridge  was  only  4  feet 
10  inches  thick,  the  upper 
three  layers  being  composed 
of  i6Xi6-inch  timbers.  The 
shape  of  this  caisson  was  an 
irregular  hexagon,  with  ex- 
treme dimensions  of  82  by 
72!  feet.  This  comparatively 
thin  roof  was  made  possible 
by  the  use  of  two  wooden 
bulkheads  below  the  roof  and 
two  iron  girders  above,  the 
latter  running  at  right 
angles  to  the  former,  and 
all  supporting  the  roof.  The 
upper  surface  of  this  roof 
was  covered  with  plate  iron, 


ART.  93 


CAISSON  ROOF   CONSTRUCTION 


286 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  VIII 


tA9Jp.---iO,&-— J 

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ART.  93 


CAISSON   ROOF   CONSTRUCTION 


287 


while  in  the  Brooklyn  bridge  caissons  the  under  side  was  cov- 
ered with  wr ought-iron  plates;  in  both  cases  this  was  done  to 
obtain  an  air-tight  roof.  It  was  a  very  expensive  method, 
since  oakum  caulking  is  sufficient.  But  in  the  Brooklyn  bridge 
caissons  it  was  done  for  the  added  purpose  of  fire  protection,  for 


288 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  VIII 


in  those  early  caissons  torches  were  used  for  lighting  purposes, 
and  as  there  was  always  a  considerable  amount  of  air  escaping 
between  the  timbers  the  danger  of  fire  was  very  great. 


El.  -5.0 


Note :  All  Posts  marked  E3  endat'Top  oftheCourses  in  which -they  a  re  shon 

Sectional          P'an: 
FIG.   930. — Pneumatic  Caisson  for  Broadway  Bridge,  Portland,  Ore. 


ART.  93 


CAISSON  ROOF   CONSTRUCTION 


289 


In  recent  years  the  tendency  has  been  to  use  more  courses  of 
3 -inch  sheathing,  usually  tongue  and  groove,  in  order  to  get 
a  more  nearly  air-tight  roof.  As  shown  in  Fig.  930  the  roof  of 


FIG.  93/. 


Showing    Inner  Showing  Braces 

Face  of  Wall  and 

and  Kneebraces  Bulkhead 

Section   A -A 
FIG.  93<7. — Quebec  Bridge  Caisson. 


Channel 
FIG.  93/*. — Section  of  Cutting  Edge 


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PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  VIII 


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ART.  93 


CAISSON   ROOF    CONSTRUCTION 


29I 


the  caisson  for  pier  4  of  the  Belief ontaine  bridge,  built  in  1892, 
consisted  of  two  courses  of  large-size  timbers,  between  which 
were  placed  two  courses  of  sheathing,  laid  diagonally.  The 
lower  side  of  the  roof  was  also  lined  with  sheathing.  Another 


FIG.  93;. — Half  Longitudinal  and  Half  Transverse  Sections. 

notable  feature  of  this  roof, 
which  is  characteristic  of 
many  built  by  Geo.  S. 
MORISON,  is  the  relatively 
thin  roof  used.  This  was 
made  possible  by  connec- 
ting the  roof  to  the  bracing 
timbers  of  the  crib  above 
by  means  of  tie  rods. 

As  shown  in  Figs.  93^,  c, 
and  d  the  rt>of  of  the  south 
main  pier  caisson  of  the 
new  Quebec  bridge  con- 
sisted of  one  solid  course 
of  longitudinal  and  one 
solid  course  of  transverse 
timbers,  separated  by  two  FlG.  93/._Framing  of  Cofferdam. 
crossed  courses  of  diagonal 

3-inch  tongue-and-grooved  planks.     Here  numerous  bulkheads 
made  possible  a  thin  roof. 


Half   End    Elevation 


Half  Section 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  VIII 


ART.  94  SIDES    OF   WORKING   CHAMBER  293 

example  of  a  reinforced-concrete  roof  is  that  for  the  caissons 
of  the  Passyunk  Ave.  highway  bridge  piers,  across  the 
Schuylkill  River,  Philadelphia.  The  largest  caisson  was  22X60 
feet  in  plan  and  its  roof  was  reinforced  with  i-inch  square, 
twisted  horizontal  rods  running  transversely  and  spaced  12 
inches  on  centers.  The  thickness  of  the  concrete  slab  first 
cast  was  18  inches,  the  forms  consisting  of  a  temporary 
wooden  ceiling  of  3Xi2-inch  planks. 

ART.  94.     SIDES  OF  WORKING  CHAMBER 

The  sides  of  the  caisson  should  be  made  strong  and  rigid 
enough,  not  only  to  take  the  direct  vertical  loads,  but  also  to 
withstand  safely  sudden  lateral  thrusts,  eccentric  loads  due  to 
unequal  sinking  of  opposite  sides,  etc.  To  prevent  leakage  of 
air  outward  and  of  water  inward  all  joints  should  be  thoroughly 
caulked.  The  necessary  thickness  of  walls  will  depend  some- 
what on  the  clear  height  of  the  working  chamber,  as  well  as  on 
the  kind  of  material  through  which  the  caisson  is  to  be  sunk. 
The  clear  height  should  not,  however,  vary  much  from  6  feet. 

The  sides  must  be  vertical.  To  batter  the  sides  for  the  pur- 
pose of  reducing  the  friction  is  to  invite  trouble.  Such  a  design 
makes  it  more  difficult  to  sink  the  caisson  plumb,  and  is  apt  to 
increase  instead  of  decrease  the  friction  by  allowing  boulders 
to  roll  into  the  open  space. 

Practically  all  working- chamber  sides  are  constructed  of 
two  forms:  namely,  that  in  which  the  vertical  section  is 
V-shaped,  and  composed  of  two  walls;  or  that  in  which  the 
vertical  section  is  essentially  a  rectangle  and  composed  of  a 
single  wall.  The  former  has  the  advantage  of  being  more 
rigid  and  so  requires  less  bracing,  while  the  latter  has  the 
advantage  of  permitting  excavation  under  the  cutting  edge 
to  be  more  easily  made. 

In  the  V-shaped  form  the  space  between  the  outer  and 
inner  walls  may  be  built  solid  with  timber,  as  was  done  in  the 
east  abutment  caisson  of  the  St.  Louis  arch  bridge;  or  it  may 
be  made  hollow  and  afterward  filled  with  concrete,  as  was  done 


294  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  VIII 

in  most  of  the  caissons  designed  by  G.  S.  MORISON,  a  typical 
form  of  which  is  shown  in  Fig.  930.  Here  the  outer  wall  was 
made  of  i2Xi2-inch  timbers,  sheathed  on  the  outside  with 
two  layers  of  planking,  the  outer  one  running  vertically  and  the 
inner  one  diagonally.  The  inner  wall  consisted  of  a  single 
thickness  of  i7Xi7~inch  timbers  sheathed  with  4-inch  planks 
and  tied  to  the  outer  wall  with  rods. 

The  St.  Louis  Municipal  bridge  caissons,  Fig.  937,  had  out- 
side walls  of  loX  1 2-inch  timbers,  sheathed  with  two  courses  of 
planking:  one  3Xi2-inch,  running  diagonally,  and  the  other, 
2 X  i2-inch,  running  vertically,  the  latter  being  on  the  outside 
to  reduce  friction  in  sinking.  The  inner  wall  wTas  formed  of 
4X1 2-inch  horizontal  planks,  stepped  and  supported  at  inter- 
vals of  10  feet  on  vertical  struts.  The  small  size  of  material 
used  in  this  wall  was  made  possible  by  reinforcing  the  concrete 
in  the  space  between  the  walls.  Stepping  the  wall  made  it 
possible  to  count  on  the  horizontal  projection  of  this  inner  wall 
as  taking  load  when  the  caisson  was  filled  with  concrete  and  in 
its  final  position.  This  cannot  be  done  when  the  wall  is  on  a 
slope.  A  further  advantage  is  that  the  projections  gave  better 
control  of  sinking,  there  being  less  danger  of  sudden  drops  than 
when  the  wall  is  sloped. 

The  rectangular  section  of  side  wall  is  used  more  widely  than 
the  triangular,  on  account  of  the  facility  with  which  the  spoil 
near  the  sides  may  be  excavated.  Figs.  930  and  /  illustrate  a 
good  example  of  this  type.  It  is  composed  of  a  double  thick- 
ness of  horizontal  12X1 2-inch  timbers,  separated  by  a  single 
thickness  of  vertical  i2Xi2-inch  timbers,  some  of  which  extend 
up  beyond  the  caisson  to  form  a  part  of  the  crib.  Both  the 
outside  and  inside  faces  of  the  wall  are  faced  with  3Xi2-inch 
planks.  Figs.  93^,  c  and  d  also  illustrate  the  same  type. 

ART.  95.    DETAILS  OF  CUTTING  EDGE 

The  cutting  edge,  as  the  part  of  the  caisson  which  rests  on 
the  ground  is  called,  must  be  designed  to  serve  four  functions: 
First,  it  must  be  sufficiently  strong  and  tough  to  stand  the 


ART.  95  DETAILS   OF   CUTTING   EDGE  295 

strains  and  abrasive  action  of  sinking;  second,  it  must  be  of  a 
form  which  will  allow  the  caisson  to  sink  readily  without 
excavating  under  the  cutting  edge;  third,  it  must  have  bearing 
surface  enough  to  prevent  sudden  sinking  when  a  soft  stratum 
is  encountered;  and  fourth,  it  should  be  so  designed  that  air 
cannot  readily  escape  under  the  same.  To  fulfill  the  first 
requirement  the  cutting  edge  is  usually  made  of  some  tough 
and  strong  wood,  such  as  elm,  or  else  is  shod  with  a  metal 
plate  or  piece  of  tough  wood.  The  second 
and  third  are  conflicting  requirements;  for 
the  second  a  true  knife  edge  is  the  ideal 
form,  while  for  the  third  a  considerable 
breadth  of  bearing  is  desirable.  As  con- 
structed, the  width  will  vary  from  about  4 
inches  to  18  inches.  To  meet  the  fourth 
requirement,  a  vertical  plate  extending 
about  6  inches  below  the  cutting  edge  is 
often  used.  Where  the  soil  is  dense  this 
plate  may  be_dispensed  with. 

Many  engineers  at  present  favor  the 
blunt  cutting  edge  in  preference  to  the  sharp  one.  T.  K. 
THOMSON'S  experience  is,  that  where  the  knife  edge  is  needed, 
i.e.,  in  hard  material,  to  allow  getting  close  to  the  outside  edge 
for  excavating,  it  would  cost  too  much  to  make  the  cutting 
edge  strong  enough,  and  where  the  material  is  soft  a  knife  edge 
is  not  needed. 

Fig.  93^  illustrates  the  use  of  a  timber  wearing  plank  on  the 
cutting  edge.  It  was  6X12  inches  in  section,  the  main  timber 
forming  the  cutting  edge  being  30X30  inches  in  section,  while 
the  upper  inner  corner  of  the  latter  was  rebated  9  inches  to 
form  a  seat  for  the  feet  of  the  vertical  wall  timbers.  The 
advantage  of  a  timber  over  a  metal  cutting  edge  lies  in  less  time 
being  required  to  obtain  it,  and  in  the  greater  ease  with  which 
it  may  be  replaced  when  broken. 

The  form  of  cutting  edge  used  by  G.  S.  MORISON,  illustrated 
in  Fig.  950,  consisted  of  a  horizontal  and  vertical  plate,  the 
latter  being  stiffened  at  intervals  and  fastened  to  the  horizontal 


296 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  VIII 


plate  by  steel  diaphragms,  which  are  stiffened  on  the  edges  by 
three  angles.  The  horizontal  plate  extended  under,  and  was 
fastened  to  both  the  lower  surf  ace  of  the  bottom  timbers  and  the 
lower  edge  of  the  outside  sheathing,  while  the  vertical  plate 
was  fastened  to  the  same  outside  sheathing.  Near  the  bottom 
the  vertical  plate  was  reinforced  with  two  others.  Fig.  930 
shows  the  appearance  of  this  cutting  edge  in  place. 

The  bottom  timber  of  the  caisson  shown  in  Fig.  93 /  extended 

out  beyond  the  timbers  above  to 
protect  the  lower  edges  of  the  out- 
side sheathing,  while  it  in  turn  was 
protected  by  steel  plates  on  all 
sides  but  the  top.  This  form  of 
construction,  having  a  vertical 
plate  on  the  outside  and  a  hori1 
zontal  angle  with  its  vertical  leg 
down  and  fastened  by  rivets  to  the 
vertical  plate,  and  with  its  hori- 
zontal leg  fastened  to  the  lower  sur- 
face of  the  lower  timber,  is  widely 
used,  but  is  not  economical. 

The  cutting  edge  of  the  caisson 
used  in  the  Kinzie  St.  draw-bridge, 
Chicago,  was  formed  with  an  8- 
inch  channel  iron  laid  horizontally 
with  flanges  turned  up  as  shown  in  Fig.  956.  The  same 
general  form  was  used  on  the  Broadway  bridge  caissons  (Fig. 
93/0,  the  only  difference  being  that  in  the  latter  case  the 
cutting-edge  timber  extended  out  to  protect  the  bottom  of 
the  sheathing,  while  in  the  former  case  the  channel  iron  served 
this  purpose.  This  form  of  metal  cutting  edge  is  the  most 
economical  and  was  designed  in  1901  by  T.  K.  THOMSON. 


Detail    of    CirH-inq    Edqe. 
(Enlarged.) 

FIG.  956. 


ART.  96.     BRACING  or  CAISSON 

Every  caisson  requires  more  or  less  bracing;  the  larger  and 
•higher  it  is  the  more  bracing  will  it  require.     This  bracing  may 


ART.  96  BRACING   OF    CAISSON  297 

be  in  the  form  of  struts  and  tiers  near  the  bottom,  running 
horizontally  the  length  and  breadth  of  the  caisson,  or  it  may 
be  in  the  form  of  bulkheads,  or  trusses.  The  latter  two 
usually  serve  the  added  purpose  of  supporting  the  roof. 

The  bracing  in  the  33  X go-foot  caisson  of  the  St.  Louis 
Municipal  bridge,  shown  in  Figs.  932'  and  /,  consisted  of  eight 
transverse  and  two  longitudinal  lines  of  horizontal  12X1 2-inch 
struts  spaced  about  10  feet  apart,  with  i}-inch  adjustable 
rods  on  both  sides  of  each  strut.  The  struts  at  their  inter- 
sections were  braced  with  vertical  12X1 2-inch  timbers  and 
pairs  of  f-inch  rods  extending  to  the  deck  of  the  caisson.  A 
similar  form  of  bracing  was  employed  in  the  Belief ontaine 
bridge  caissons,  as  illustrated  in  Fig.  930,  as  well  as  in  the 
Broadway  bridge  caissons,  Figs.  930  and/. 

The  south  main  pier  caisson  of  the  New  Quebec  bridge, 
55X180  feet  in  plan,  was  divided  by  timber  bulkheads,  as 
shown  in  Figs.  936  and  c?  into  eighteen  rectangular  compart- 
ments approximately  19X25  feet  in  size.  These  longitudinal 
and  transverse  bulkheads  were  respectively  24  and  12  inches 
thick,  except  the  lower  course  which  was  12  inches  thicker. 
All  extended  from  the  ceiling  to  about  the  top  of  the  cutting 
edge.  Each  transverse  bulkhead  was  trussed  by  a  pair  of 
adjustable  diagonal  rods,  the  ends  of  which  took  bearing  in  the 
end  walls  at  roof  level,  through  beveled  washers;  in  the  center 
they  bore  on  steel  plates,  the  latter  in  turn  bearing  on  both 
longitudinal  and  transverse  bulkheads.  The  end  walls  on 
each  side  of  the  longitudinal  bulkhead,  were  braced  by  a 
solid- web  knee  brace  1 2  inches  thick,  reaching  from  the  cutting 
edge  to  the  top  of  the  first  transverse  bulkhead.  Between 
bulkheads  the  sides  were  knee-braced  to  the  roof  by  single  and 
double  i2X i2-inch  struts  inclined  at  an  angle  of  45  degrees. 

The  bulkheads  of  the  east  abutment  raisson  of  the  St.  Louis 
arch  bridge  were  of  very  massive  construction,  being  made  of 
eight  horizontal  courses  of  timber,  the  upper  course  having 
eight  timbers  in  it,  making  a  width  of  io  feet,  while  the  bottom 
course  had  three  timbers,  making  a  width  of  3^  feet.  The 
numbers  varied  in  the  horizontal  courses  between  these  two 


298  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  Vlft 

values  in  such  a  way  as  to  give  a  V-shaped  section  of  bulkhead. 
The  height  was  9  feet. 

A  longitudinal  wooden  truss  was  used  to  brace  the  31 X  79- 
foot  caisson  of  the  Havre  de  Grace  bridge.  It  was  6  feet  deep, 
the  upper  and  lower  chords  being  composed  of  two  pieces  of 
i2X i2-inch  timbers.  The  web  members,  both  vertical  and 
diagonal,  were  composed  of  timber  struts  and  diagonal  rods,  the 
latter  extending  through  the  first  deck  course  of  the  caisson. 
Cross  braces  were  placed  between  the  bottom  chord  of  the 
truss  and  the  side  walls. 

ART.  97.     CRIB  CONSTRUCTION 

Some  writers  consider  the  crib  as  a  part  of  the  caisson,  but 
since  the  crib  may  sometimes  be  dispensed  with  and  the  pier 
built  directly  on  the  caisson,  it  will  avoid  confusion  by  separat- 
ing the  two.  A  certain  height  of  crib  is  often  built  as  an  integral 
part  of  the  caisson  to  facilitate  floating  the  structure  into  place. 
The  purpose  of  the  crib  is  two-fold:  First,  it  serves  as  a  form 
for  the  concrete;  and  second,  it  serves  temporarily  as  a  coffer- 
dam to  keep  out  the  water.  If  the  masonry  or  concrete  work  is 
kept  sufficiently  in  advance  of  the  sinking  the  crib  may  some- 
times be  dispensed  with,  but  this  is  sledom  done  because  it 
brings  too  much  weight  on  the  caisson.  The  crib  is  a  per- 
manent part  of  the  foundation  and  usually  its  walls  are  a  con- 
tinuation of  the  walls  of  the  caisson,  perhaps  slightly  modified. 
The  crib  is  thoroughly  braced  with  longitudinal  and  transverse 
timbers  left  permanently  in  place. 

Although  it  is  customary  to  fill  the  crib  with  concrete,  yet 
under  some  circumstances  this  may  not  be  done.  In  the 
substructure  for  Pier  2  of  the  Memphis  bridge,  where  the  na- 
ture of  the  soil  made  it  necessary  that  the  load  on  the  founda- 
tion bed  be  kept  down  to  a  minimum,  the  pockets  near  the 
walls  in  the  crib  were  left  empty,  while  for  about  15  feet  down 
from  the  top  of  the  crib  a  solid  timber  grillage  was  used,  thus 
decreasing  the  weight  of  the  structure  very  considerably. 

The  crib  for  the  south  main  pier  of  the  New  Quebec  bridge 


ART.  97  CRIB   CONSTRUCTION  299 

had  a  wall  made  of  a  single  thickness  of  horizontal  i2Xi2-inch 
timbers  to  a  distance  of  25  feet  above  the  cutting  edge  of  the 
caisson,  braced  by  inside  vertical  i2Xi2-inch  timbers,  spaced 
as  shown  in  Figs.  936  and  c,  the  latter  being  extensions  of  certain 
of  the  vertical  timbers  forming  the  sides  of  the  caisson.  The 
outside  was  sheathed  with  the  same  material  as  used  for  the 
caisson.  The  walls  were  braced  with  horizontal  longitudinal 
and  transverse  struts  24  inches  apart  vertically,  up  to  a  height 
of  25  feet  above  the  cutting  edge  of  the  caisson,  dividing  the 
crib  into  ninety  pockets  approximately  10  feet  square.  A 
similar  bracing  course  was  placed  29  feet  above  the  cutting 
edge  of  the  caisson;  above  this  point  there  was  no  bracing,  it 
being  replaced  with  a  concrete  retaining  wall  reaching  to  the 
top  of  the  crib,  built  against  the  walls  of  the  latter  and  battered 
on  the  interior  face,  increasing  in  thickness  from  the  top  down. 
This  was  placed  early  in  order  to  allow  it  to  harden  before  any 
stress  was  put  upon  it.  The  advantage  of  this  retaining  wall  is 
that  it  made  the  upper  part  of  the  crib  a  solid  monolithic  mass  of 
concrete. 

The  crib  shown  in  Fig.  93 a  had  the  bracing  carried  to  the  top 
and  was  notable  on  account  of  the  manner  in  which  the  bracing 
was  tied  together  with  vertical  rods.  Here  the  lower  courses 
of  bracing  helped  to  carry  the  roof  loads;  for  this  reason  the 
part  of  the  crib  up  to  the  top  of  the  rods  passing  through  the 
roof  may  be  considered  a  part  of  the  caisson. 

The  walls  of  the  cribs  for  the  St.  Louis  Municipal  bridge 
piers  consisted  for  the  most  part  of  one  thickness  of  loX  1 2-inch 
timbers,  sheathed  on  the  outside  with  one  layer  of  3-inch  diag- 
onal and  one  layer  of  3-inch  vertical  planks.  The  bracing 
consisted  of  vertical  i2Xi2-inch  timbers  and  of  eight  rows  of 
horizontal  transverse  and  two  of  horizontal  longitudinal  loX  12- 
inch  timbers.  As  shown  in  Fig.  93^'  a  large  amount  of  3X10- 
inch  diagonal  bracing  was  also  used,  giving  a  truss-like  action 
to  the  bracing  and  greatly  strengthening  it. 

The  crib  construction  of  the  Broadway  bridge  is  shown  in 
Figs.  930  and/;  the  detail  are  so  simple  that  no  explanation  is 
necessary. 


300  PNEUMATIC   CAISSONS  FOR  BRIDGES  CHAP.  VIII 

ART.  98.     COFFERDAM  CONSTRUCTION 

Both  durability  and  appearance  require  that  no  part  of 
the  crib  extend  above  low- water  level;  and  moreover,  to  keep 
the  obstruction  to  the  current  as  small  as  possible,  the  crib  is 
stopped  and  the  pier  commenced  at  a  considerable  distance 
below  low  water.  In  some  cases,  where  the  current  has  a  high 
velocity,  the  pier  is  started  at  or  below  the  river  bed,  or  the  upper 
part  of  the  crib  is  built  with  pointed  ends.  For  these  reasons, 
unless  conditions  are  such  that  the  pier  construction  can  be 
kept  well  above  water-level,  a  cofferdam  in  which  to  build  the 
pier  becomes  necessary.  Ordinarily  cofferdams  may  be  dis- 
pensed with  only  when  the  construction  is  carried  on  at  low 
water  stages  or  when  the  friction  and  resistance  to  sinking  is 
large.  As  a  general  rule  it  is  desirable  to  keep  the  weight  on  the 
caisson  as  small  as  possible  as  this  affords  better  control  of 
the  sinking.  Even  when  possible  many  engineers  prefer  not 
to  start  building  the  pier  until  the  caisson  is  sunk  to  final  position, 
for  only  at  such  a  time  can  the  masonry  be  started  in  the  correct 
position.  The  walls  of  the  cofferdam  are  usually  made  of 
lighter  construction  than  those  of  the  crib,  but  it  is  always 
thoroughly  caulked,  and  braced  by  struts  running  the  length 
and  breadth  of  the  structure.  As  the  pier  is  built  up  these  braces 
are  removed  and  the  walls  are  braced  against  the  pier.  On  the 
completion  of  the  latter  the  cofferdam  is  removed,  if  not  the 
whole  structure,  at  least  that  part  above  low  water. 

Figs.  93  &  and  /  illustrate  the  cofferdam  used  for  one  of  the 
piers  of  the  St.  Louis  Municipal  bridge.  The  left  dotted  lines 
represent  the  top  course  of  crib  and  the  right  dotted  lines  the  top 
of  struts.  The  cofferdam,  which  was  33  feet  ;|  inches  long, 
consisted  of  a  frame  of  horizontal  6  X  8-inch  and  vertical  6X6- 
inch  timbers,  sheathed  with  2Xi2-inch  planks.  It  was  braced 
with  6  X  8-inch  struts,  4  feet  apart  vertically,  and  in  rows  about 
10  feet  apart  horizontally. 

The  cofferdam  used  for  the  Brooklyn  pier  of  the  Manhattan 
bridge,  New  York,  N.  Y.,  was  one  of  the  highest  that  has  ever 
been  used  in  pneumatic  caisson  work,  being  44  feet  high  and 


ART.  99  PNEUMATIC   CAISSONS   OF   CONCRETE  301 

about  75X144  feet  in  plan.  It  was  built  in  three  sections, 
the  sides  of  the  first  two  sections  being  made  of  10X1 2-inch 
horizontal  timbers  laid  close  and  supported  by  i2Xi8-inch 
verticals,  spaced  12  feet  apart.  On  the  outside  two  layers  of 
3 X i2-inch  sheathing  were  placed,  the  inner  planking  being 
horizontal  and  the  outer  vertical.  The  upper  section  differed 
from  the  others  only  in  having  8X1 2-inch  instead  of  loX  12- 
inch  horizontals. 

ART.  99.     PNEUMATIC  CAISSONS  or  CONCRETE 

Pneumatic  caissons  built  entirely  of  concrete  have  been  used 
to  some  extent  in  Europe,  but  in  this  country  the  nearest 
approach  to  the  all-concrete  pneumatic  caisson  are  those  for  the 
Beaver  bridge,  described  in  Art.  90.  As  there  explained  most 
of  the  sinking  was  done  by  the  open-well  method.  With  the 
exception  of  a  very  few  cases,  like  the  one  just  noted,  the 
tendency  in  this  country  has  been  to  use  wood,  but  at  the  same 
time  to  decrease  the  amount  formerly  used  by  reinforcing  the 
lower  part  of  the  crib  concrete,  as  was  done  in  the  St.  Louis 
Municipal  bridge  caissons.  A  covering  of  timber  offers  three 
advantages:  First,  it  avoids  the  necessity  of  waiting  for  the 
concrete  to  harden  before  commencing  sinking  operations; 
second,  it  offers  less  resistance  to  sinking  because  of  the  reduced 
friction  on  the  sides;  and  third,  it  forms  a  protection  in  sinking 
for  the  concrete  of  the  sides. 

The  pneumatic  process  was  used  during  the  final  part  of  the 
sinking  of  the  Beaver  bridge  caissons  in  order  that  the  bottom 
might  be  thoroughly  cleaned,  as  well  as  to  permit  laying  the 
concrete  filling  in  air.  The  caisson  was  changed  from  the  open 
to  the  pneumatic  type  in  the  following  manner:  It  was  first 
freed  of  water  down  to  a  level  which  permitted  the  placing  of 
horizontal  wooden  frames  in  each  of  the  wells  at  an  elevation 
of  about  9  feet  above  the  cutting  edge.-  Concrete  was  then 
placed  on  these  forms,  filling  the  wells,  the  first  7  feet  being 
allowed  to  harden  for  a  week  before  placing  the  rest.  At  the 
center  of  each  well  a  vertical  shaft,  3  feet  in  diameter,  was 


302  PNEUMATIC   CAISSONS  FOR  BRIDGES  CHAP.  VIII 

placed  to  form  a  means  of  communication  between  the  working 
chamber  and  the  outside. 

ART.  100.     PNEUMATIC  CAISSONS  OF  METAL 

The  abundance  of  timber  in  America  has  limited  the  use  of 
the  metal  type  to  relatively  few  cases,  while  in  Europe  it  has 
been  extensively  used. 

The  river  piers  of  the  St.  Louis  arch  bridge,  the  first  structure 
in  this  country  founded  on  large  pneumatic  caissons,  rest  on 
metal  caissons.  Two  reason  may  be  given  for  this  fact:  First, 
there  was  considerable  uncertainty  as  to  the  action  of  a  timber 
roof  when  subjected  to  the  horizontal  thrust  from  the  super- 
structure; and  second,  timber  had  not  been  used  in  caisson 
construction  to  serve  as  a  precedent. 

The  caisson  for  the  east  pier,  which  was  hexagonal  in  plan, 
with  over-all  dimensions  of  60X82  feet,  had  walls  of  wrought- 
iron  plates  f  inch  thick,  braced  with  iron  brackets  extending 
from  the  bottom  to  the  top,  and  spaced  2\  feet  apart.  The  roof 
was  formed  of  ^-inch  iron  plates  riveted  to  the  lower  flanges 
of  thirteen  parallel  iron  girders,  spaced  5  feet  6  inches  apart. 
It  was  also  supported  by  two  heavy  bulkheads  of  oak  timber, 
7  feet  high,  in  the  air  chamber.  These  strong  supports  for  the 
roof  were  necessary  because  the  latter  had  to  take  the  entire 
weight  of  a  loo-foot  height  of  stone  masonry. 

The  walls  of  the  caisson  extended  above  the  roof  to  form  an 
enclosure,  in  which  the  masonry  was  laid.  No  monolithic  con- 
crete was  used  in  this  structure.  For  some  distance  up  the 
masonry  covered  the  entire  cross-section  of  the  crib,  but  above 
this  it  was  stepped  off,  the  space  between  the  iron  envelope  and 
the  masonry  being  braced  with  timbers  and  filled  with  sand. 
For  the  west  pier  caisson  the  iron  envelope  was  carried  up  but 
20  feet,  after  which  the  masonry  was  laid  in  the  open,  care 
being  taken  to  keep  the  top  of  the  same  above  water-level. 

The  metal  pneumatic  caissons  for  the  Alexander  III  bridge, 
Paris,  France,  built  in  1897,  are  among  the  largest  of  any  type 
ever  used.  In  plan  one  caisson  had  the  shape  of  a  parallelogram 


ART.  100  PNEUMATIC   CAISSONS   OF   METAL  303 

(the  angle  being  84  degrees),  the  length  of  the  sides  being 
approximately  145  and  no  feet,  transversely  and  parallel, 
respectively,  to  the  axis  of  the  bridge.  The  working  chamber 
had  a  clear  height  of  6.23  feet  and  through  this  extended  four 
transverse  girders,  each  6.23  feet  high,  their  bottoms  forming 
cutting  edges,  and  dividing  the  chamber  into  five  subchambers. 
On  their  upper  flanges  these  girders  supported  twenty-seven 
longitudinal  girders,  5.2  feet  deep,  which  carried  the  roof  of  the 
steel-plate  platform  that  formed  the  deck  of  the  caisson  proper. 
The  transverse  girders  had  solid-plate  webs  for  nearly  one-third 
of  their  length  at  each  end  and  open  web  members  in  the  central 
part.  The  longitudinals  were  ordinary  latticed  girders.  The 
working  chamber  had  a  roof  of  steel  plates  o.  2  inch  thick  which 
were  fastened  to  the  lower  flanges  of  the  longitudinal,  and  to  the 
upper  flanges  of  the  transverse  girders.  These  plates  did  not 
extend  horizontally  through  to  the  vertical  sides  of  the  caisson, 
but  at  the  sides  followed  down  the  inclined  end  posts  of  the 
transverse  girders,  and  at  the  ends  followed  the  knee  braces 
down  to  the  cutting  edge  to  give  sloping  inside  walls  on  all 
four  sides. 

Between  these  inclined  plates  and  the  outer  vertical  walls 
was  a  triangular  space  filled  with  concrete.  The  outside  wall 
plates  and  the  transverse  girders  were  all  stiffened  with 
knee  braces  extending  from  the  cutting  edge  to  the  longi- 
tudinal girders. 

The  outside  wall  plates  were  reinforced  on  the  lower  edges  by 
an  outside  vertical  plate  and  the  vertical  flange  of  an  inner  angle, 
while  the  transverse  girders  were  reinforced  for  bearing  and 
cutting  strains  by  adding  two  angles  riveted,  with  their  hori- 
zontal flanges  upward,  to  the  lower  edge  of  the  vertical  web 
plate  of  the  lower  chord.  The  cofferdam  above  was  19.7  feet 
high  and  was  composed  of  riveted  and  caulked  vertical  plates, 
o.  1 18  inch  thick,  with  a  light  angle-iron  frame  and  light  inclined 
angle-iron  struts  from  near  the  upper  edge  and  the  middle  of 
the  top  of  the  transverse  girders.  The  total  distance  sunk  was 
27  feet  below  ordinary  water-level.  For  further  details  the 
reader  is  referred  to  either  Engineering  News,  vol  39,  page  254, 


304  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  VIII 

April  •  21,    1898,   or  Engineering  Record,   vol.   37,   page   275, 
Feb.  26,  1898. 

ART.  101.     CYLINDER  PIER  CAISSONS 

The  foundation  for  a  cylinder  pier  is  often  placed  by  the 
pneumatic  process,  in  which  case,  like  the  open-cylinder  caisson, 
there  is  usually  no  particular  point  at  which  the  caisson  may  be 
said  to  end  and  the  pier  begin.  The  pneumatic  cylinder  caisson 
is  very  similar  to  the  open  caisson  in  many  cases,  the  only 
difference  being  that  the  former  is  fitted  with  horizontal  dia- 
phragm doors  to  form  the  air-lock.  Often  a  part  of  the  sinking 
is  done  by  the  open-caisson  method  and  the  remainder  by  the 
pneumatic  method.  As  noted  in  Art.  92  the  cylinder  caisson 
was  the  first  type  of  foundation  to  which  the  pneumatic  process 
of  sinking  was  applied  in  this  country. 

Fig.  ioia  illustrates  the  cylinder  piers  and  pneumatic  cylinder 
caissons  used  for  the  Columbia  River  bridge  at  Trail,  B.  C. 
The  shells  were  of  steel  plates  from  fV  to  rV  inch  thick.  The 
lower  6 1  feet  were  formed  of  a  double  shell,  the  diameter  of 
the  inner  shell  being  3  feet,  and  that  of  the  outer  one  9  feet  at 
the  bottom  and  6  feet  at  the  top.  Beginning  at  a  point 
8  feet  above  the  bottom  of  the  caisson  the  inner  shell  .was 
splayed  out  to  meet  the  outer  shell  at  the  cutting  edge,  thus 
forming  a  working  chamber  8  feet  high.  Near  the  bottom 
the  two  shells  were  braced  together  with  diagonal  lacing  as 
shown  in  the  diagram. 

The  upper  parts  of  the  cylinders  were  connected  and 
braced  by  two  vertical  transverse  rVX6o-inch  plates,  2  feet 
apart,  braced  together  and  the  space  between  the  two  filled 
with  concrete. 

The  air-lock  was  formed  by  placing  two  diaphragm  doors  in 
the  inner  shaft,  one  about  13  feet  above  the  cutting  edge  and 
the  other  at  a  point  about  16  feet  higher.  As  sinking  proceeded, 
a  third  door,  about  16  feet  above  the  second  door,  was  added, 
the  upper  two  doors  being  used  to  form  the  lock,  while  the  lower 
door  was  used  for  emergencies.  These  caissons  were  designed 


ART.  101 


CYLINDER   PIER   CAISSONS 
I 


305 


Bracing       Frame  Bottom  of  Pier  showing  Web 

FIG.   loia. — Pneumatic  Cylinder  Caissons,  Trail,  B.  C. 


20 


3°6 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  VIII 


by  WADDELL  &  HARRINGTON,  and  may  be  considered  to  repre- 
sent current  standard  practice. 

In  the  repairs  of  the  Atchafalaya  River  bridge,  each  pier 
consisted  of  a  pair  of  8-foot  diameter  steel  cylinders,  filled  with 
concrete  and  braced  together  at  the  top  by  a  stiffened  web 
plate  or  diaphragm  about  20  feet  high,  as  shown  in  Fig.  loib. 
Each  cylinder  had,  in  addition  to  the  outer  8-foot  diameter 


'  Stee!. 


FIG.   1016. — Pneumatic  Cylinder  Caissons,  Atchafalaya  River  Bridge. 

shell,  an  inner  concentric  shell  $  feet  in  diameter,  with  a 
conical  section  uniting  it  with  the  cutting  edge  and  closing  the 
lower  end  of  the  annular  space  between  the  two  shells.  The 
shells  were  connected  by  four  stiff  webs.  The  inside  shell 
terminated,  about  22^  feet  below  the  top  of  the  outer  one,  the 
latter  having  a  total  length  of  over  135!  feet  and  was  made  with 


ART.  102  COMBINATION   CYLINDER   CAISSONS  307 

5-foot  rings  erected  in  lo-foot  sections.  The  working  chamber 
was  25  feet  high,  and  had  a  roof  consisting  of  a  2-foot  oak 
diaphragm  made  of  four  thicknesses  of  timber,  with  a  circular 
hole  2  feet  in  diameter  closed  by  a  cast-iron  door. 

In  the  piers  of  the  Glasgow  bridge,  which  were  sunk  by  the 
pneumatic  process  the  diameter  of  the  outer  shell  was  15  feet, 
the  thickness  of  the  shell  at  the  base  being  |  inch  and  at  the  top 
T5^  inch.  The  shaft  which  was  3  feet  7  inches  in  diameter 
formed  the  inner  cylinder,  and  this  was  removed  before  filling 
the  working  chamber  and  air-shaft. 

Almost  no  records  exist  of  the  use  of  the  reinforced-concrete 
pneumatic  cylinder  caisson.  In  Art.  102  there  is  given  an 
example  of  this  type,  in  which  the  first  part  of  the  sinking  was 
done  by  the  open-caisson  method  and  the  latter  part  by  the 
pneumatic  process. 

ART.  102.     COMBINATION  CYLINDER  CAISSONS 

With  the  cylinder  caisson  it  is  a  simple  matter  to  construct 
the  cylinder  to  be  used  either  as  an  open  or  a  pneumatic 
caisson.  This  makes  it  possible  to  utilize  the  advantages  of 
both  methods  of  sinking,  the  open  caisson  being  used  for  that 
part  of  the  sinking  in  which  the  material  can  be  dredged  or 
pumped  out,  and  the  pneumatic  process  for  that  part  where 
boulders  or  compact  material  is  met  with,  and  in  finally  prepar- 
ing the  foundation  bed  and  placing  the  concrete  filling  in  the 
working  chamber. 

The  caissons  for  the  Merrimac  River  bridge,  between 
Salisbury  and  Newburyport,  Mass.,  were  of  this  type,  Each 
caisson  consisted  of  an  8-foot  diameter  cast-iron  shell,  the 
metal  being  i|  inches  thick  and  cast  in  8-foot  sections.  These 
sections  had  inside  flanges  bolted  together  and  a  mixture  of  red 
lead  and  linseed  oil  was  placed  between  the  joints. 

The  cylinders  were  sunk  by  inside  dredging  to  a  layer  of 
boulders  and  gravel.  They  were  then  loaded  with  pig  iron, 
air-locks  placed  on  top,  and  air  pressure  applied.  No  attempt 
was  made  to  sink  the  caissons  through  the  boulders,  but  instead 
a  novel  method  was  used  to  transform  this  boulder  and  gravel 


3o8 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  VIII 


layer  into  a  good  foundation  bed.  The  pressure  in  the  cylinder 
was  reduced  a  little  allowing  about  a  foot  or  more  of  water  to 
rise.  Portland  cement  was  then  mixed  with  the  water  to  form 
a  grout,  which  was  kept  well  stirred  while  the  air  pressure  was 
increased  to  force  the  grout  into  the  gravel.  On  completion  of 
the  grouting  a  depth  of  from  10  to  20  feet  of  1-2-4  concrete 
was  laid  under  air  pressure,  and  allowed  to  harden,  after  which 
the  remainder  was  laid  in  the  open. 


FIG.   io2a. — Pneumatic  Caissons  of   Reinforced   Concrete  for   Bronx   Viaduct  of 
New  York  Connecting  Railway 

Fig.  1020  shows  the  main  details  of  concrete  cylinder  caissons 
used  for  foundations  of  the  Bronx  viaduct  of  the  New  York 
Connecting  Railway.  The  caissons  varied  from  10  to  18  feet 
in  diameter  and  were  sunk  to  a  maximum  depth  of  55  feet. 
The  cutting  edge  was  formed  of  a  steel  angle  and  steel  plate,  and 
the  concrete  composing  the  caisson  was  well  reinforced  with 
vertical  and  horizontal  rods.  When  sinking  through  clay  the 
open  dredging  process  was  used,  while  in  passing  through 
quicksand  air-locks  were  placed  in  the  upper  part  of  the 
shafts  and  the  pneumatic  process  used. 


CHAPTER  IX 
PNEUMATIC  CAISSONS  FOR  BRIDGES 

ART.  103.     SHAFTS  AND  AiR-Locxs 

The  shafts,  which  form  the  means  of  communication  between 
the  working  chamber  and  the  outside,  are  circular  in  shape  and 
in  most  cases  are  of  steel  plate  f-inch  thick;  and  in  sections 
about  10  feet  long,  each  section  being  flanged  and  bolted  to  the 
one  above  and  below.  Separate  "shafts  are  ordinarily  used  for 
men  and  materials,  those  for  the  men  being  about  3  feet  in 
diameter,  although  if  an  elevator  is  used  they  are  often  as  large 
as  6  feet  in  diameter.  The  shafts  for  the  removal  of  spoil  are 
about  2  feet  in  diameter.  Where  the  depths  are  only  moderate 
it  is  customary  to  have  a  ladder  built  in  the  shaft  used  by  the 
men,  but  when  the  depth  is  considerable  a  power  elevator  should 
always  be  employed  as  it  is  extremely  exhausting  to  climb  a 
long  distance  after  working  under  high  pressure.  The  men 
often  use  the  excavating  bucket  as  an  elevator. 

As  explained  in  Art.  92  the  air-lock  is  a  chamber  having  two 
doors,  one  of  which  opens  to  the  atmosphere  and  the  other  to 
the  working  chamber.  These  doors  are  so  placed  that  the 
unequal  air  pressure  will  always  force  them  against  their  seats, 
which  have  rubber  gaskets  to  prevent  the  escape  of  air.  The 
operation  of  the  lock  for  men  is  as  follows:  The  lower  door 
being  closed  and  the  upper  one  open,  a  man  enters;  the  upper 
door  is  then  closed  and  compressed  air  slowly  admitted  to  the 
lock,  and  as  soon  as  the  pressure  in  it  becomes  equal  to  that 
below,  the  lower  door  opens  allowing  the  man  to  enter  the 
working  chamber. 

The  air-lock  may  be  of  any  shape  and  of  any  desired  size, 
the  latter  depending  on  the  number  of  men  or  the  amount  of 
material  it  is  desired  to  lock  through  at  a  time.  The  material 
Jock  is  often  but  a  section  of  the  shaft. 

3C9 


3io 


PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 


A— 


Front  Elevation, 


Section  through  Center. 


Section  A-B. 

FIG.   loja. — Material  Lock  used  in  Pneumatic  Caissons  of  Memphis  Bridge,  1891. 


ART.  103 


SHAFTS    AND   AIR-LOCKS 


311 


In  the  early  caissons  the 
lock  was  placed  at  the  bot- 
tom of  the  shaft  and  ex- 
tended down  into  the  work- 
ing chamber,  but  at  pres- 
ent   the   material    lock  is 
always  placed  at  the  top  of 
the  shaft,  while  the  man 
lock  is  placed  either  at  the 
top  or  some  distance  up 
from  the  bottom.     Caisson 
sinking   with   the  lock   at 
the  bottom  is  a  risky  un- 
dertaking because  a  '  blow- 
out/ that  is,  a  sudden  out- 
rush  of  air,   will  cause  a 
like    inrush    of   water   ac- 
companied by  a  rapid  sink- 
ing of  the  caisson,  which 
is  almost  sure  to  damage 
the  lock.     With   the  lock 
out  of  commission  the  men 
in   the   working    chamber 
have  no  chance  to  escape, 
while  if  the  lock  is  at  the 
top  the  men  can  climb  up 
and  take  refuge  in  the  shaft 
above    the    level    of    the 
water.      About    the    only 
disadvantage  in  having  the 
lock  on  top  of  the  shaft  lies 
in  the  necessity  of  remov- 
ing it  each  time  a  new  sec- 
tion is  added  to  the  shaft; 
but  with  properly  designed 
connections  this  can  easily 
be  done,  and  without  dan- 


Vertical 

Section. 


Sectional     Plan . 

FIG.   1036. — Air  Lock  for  Men,  Memphis 
Bridge. 


3I2 


PNEUMATIC   CAISSONS   FOR  BRIDGES 


CHAP.  IX 


ger,  by  having  an  auxiliary  door  fitted  to  the  lower  end  of  the 
shaft  in  the  roof  of  the  working  chamber  which  is  closed 
when  the  lock  is  taken  off. 

Two  forms  of  air-locks  extensively  employed  for    caissons 
used   for    the   foundations   of   buildings   are   illustrated    and 


FIG.   lose. — Arrangement  of  Air  Lock,  Shafts,  Pipes,  etc.     Bellefontaine  Bridge- 

described  in  Art.  119.  The  particular  advantage  which  these 
types  possess  is  that  the  bucket  may  be  lowered  into  the  air 
chamber,  filled  and  taken  out  without  detaching  from  the 
hoisting  rope. 

Another  form  of  material  lock  which  has  been  employed  is 
illustrated  in  Fig.  103^  this  particular  one  being  used  on  the 
Memphis  bridge  caissons.  The  method  of  operation  is  de- 
scribed in  Art.  107.  The  essential  difference  between  this 


ART.  104  DESIGN   OF   CAISSONS  313 

and  the  types  described  in  Art.  1 19  lies  in  the  fact  that  here  the 
upper  door,  instead  of  being  in  a  horizontal  plane,  lies  in  a 
vertical  plane  at  B.  This  necessitates  either  dumping  the 
material  out  on  being  brought  to  the  top  or  else  the  bucket  must 
be  detached  from  the  cable  and  taken  out. 

The  form  of  lock  for  men  employed  on  the  above  mentioned 
bridge  is  illustated  in  Fig.  1036.  It  is  shown  in  position  in 
Fig.  1030.  laThe  upper  shaft  through  which  the  elevator- cage 
runs  is  a  cylinder  6  feet  in  diameter,  the  air-lock  itself  is  a 
cylinder  6  feet  in  diameter,  and  the  shaft  leading  to  the  caisson, 
a  cylinder  4  feet  in  diameter;  the  three  cylinders  are  tangent  to 
each  other,  and  the  shells  are  connected  by  cast-iron  door  frames 
carrying  doors,  while  a  fourth  door  opening  outward  was 
placed  at  the  bottom  of  the  lower  shaft;  in  working,  the  door 
between  the  two  shafts  was  always  kept  closed,  and  the  door 
at  the  bottom  of  the  bottom  shaft  was  always  left  open;  it  was 
possible,  however,  if  an  emergency  had  arisen  to  use  the  lower 
section  of  the  shaft  as  an  air-lock  in  itself;  when  the  filling  of  the 
working  chamber  was  completed  the  bottom  door  was  per- 
manently closed." 

ART.  104.    DESIGN  OF  CAISSONS 

It  is  impossible  to  compute  even  approximately  the  stresses 
in  the  various  parts  of  a  caisson  and  for  this  reason  it  is  best 
largely  to  follow  precedent.  Engineers  who  are  experts  on 
caisson  work,  have  built  many  caissons  and  by  observing  the 
weak  points  have  developed  strong  structures  with  increasing 
economy.  The  examples  given  in  the  preceding  articles  are 
representative  of  the  best  forms  in  use,  and  are  recommended 
to  the  careful  consideration  of  engineers  interested  in  this 
subject.  For  more  extended  information  the  reader  is  referred 
to  the  bibliography  in  Chap.  XIX. 

T.  K.  ThoMSON,  a  consulting  engineer  who  has  specialized  in 
pneumatic  caissons,  writes  on  their  design  as  follows: 

1  The  Memphis  Bridge,  by  GEO.  S.  MORISON. 


314  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 

l"It  is  necessary  to  use  considerable  common  sense  and 
experience  in  attempting  to  calculate  the  strains  in  a  caisson. 
As  regards  the  deck,  for  example,  it  is  very  easy  to  calculate 
the  weight  to  be  carried  by  the  deck  and  the  strains  that  would 
result  therefrom,  and  we  know  that  the  air  pressure  acting  up 
against  the  roof  will  counterbalance  a  great  deal  of  this  weight, 
making  it,  in  fact,  something  like  a  pontoon  floating  in  the 
water.  But  on  the  other  hand,  the  air  pressure  is  often  slacked 
down  to  almost  nothing  in  order  to  overcome  the  friction,  and  is 
raised  again  before  much  water  has  time  to  enter  the  working 
chamber;  and  sometimes  an  accident  to  the  air  plant  will 
suddenly  cut  off  the  supply  of  air,  throwing  a  tremendous  strain 
on  the  roof.  If  the  principal  weight  on  the  roof  is  concrete  it 
will  in  many  cases  be  self-sustaining  unless  too  fresh. 

"The  same  with  the  sides.  If  the  material  were  absolutely 
homogeneous  all  around  and  the  caisson  were  sunk  absolutely 
plumb,  which  almost  never  happens,  and  the  air  pressure  were 
kept  just  equal  to  the  outside  pressure,  then  we  wpuld  have 
practically  no  strain  on  the  sides — but  all  practical  caisson  men 
have  seen  the  sides  of  caissons  collapse,  and  some  very  strongly 
built  ones  at  that.  A  very  much  more  frequent  cause  of 
accident  than  loss  of  air  pressure  is  to  strike  some  obstruction 
on  one  side,  deflecting  the  cutting  edge,  and  thus  throwing 
much  of  the  weight  of  the  caisson  on  the  weakened  side,  making 
bad  worse.  .  .  . 

"in  building  wooden  caissons  I  very  seldom  halve  the  timbers 
or  use  dovetailed  joints,  preferring  to  use  butt  joints  as  much  as 
possible  with  plenty  of  drift  bolts.  The  trouble  with  butt  joints, 
however,  is  that  while  a  carpenter  will  make  a  dovetail  or 
half-lap  joint  fit  he  will  probably  leave  an  inch  or  so  play  in  a 
butt  joint. 

"The  deck  timbers,  as  well  as  those  in  the  sides,  should  be 
planed  on  one  side  and  one  edge,  for  the  sizes  would  otherwise 
vary  too  much  to  get  a  good  job,  while  the  planking  for  the 
outside  and  inside  of  the  air  chamber  should  be  either  tongue 
and  groove,  or  the  sides  should  be  planed  for  a  caulking  joint. 

1  See  "Construction,"  Nov.,  1908. 


ART.  105  BUILDING   AND   PLACING   THE   CAISSON  315 

The  plank  should,  of  course,  have  its  faces  also  planed." 
Since  very  many  drift  bolts  are  required  in  fastening  together 
the  heavy  timbers  in  wooden  caisson  construction,  it  is 
desirable  to  adopt  the  proper  diameter  of  holes  to  be  bored. 
For  the  results  of  experiments  on  the  holding  power  of  drift 
bolts  and  the  best  ratio  of  the  diameter  of  hole  to  that  of 
bolt,  see  Art.  10  in  JACOBY'S  Structural  Details. 

ART.  105.    BUILDING  AND  PLACING  THE  CAISSON 

The  caisson  may  be  built  on  ways  on  the  shore;  on  pontoons 
anchored  near  the  shore,  or  over  the  site  where  it  is  to  be  sunk; 
or  on  a  temporary  platform  supported  by  piles.  Of  the  three 
methods,  building  on  ways  on  the  shore  is  the  most  widely  used, 
but  to  make  this  method  satisfactory  the  following  conditions 
must  obtain:  First,  there  must  be  deep  water  near  the  shore; 
second,  the  soil  must  be  sufficiently  firm  to  hold  the  caisson, 
either  with  or  without  the  use  of  bearing  piles;  third,  there 
must  be  no  danger  of  a  high  and  rapid  rise  in  the  river; 
and  fourth,  the  shore  must  not  be  at  a  great  distance  from 
the  site  of  sinking. 

Where  satisfactory  shore  conditions  do  not  obtain  and  where 
the  water  is  deep  and  subject  to  sudden  rises  the  pontoon 
method  is  the  best.  Where  the  depth  of  water  is  not  great  and 
where  the  river  is  not  subject  to  considerable  changes  of  level 
the  method  of  using  a  temporary  platform  on  piling  is  con- 
venient. Caissons  for  abutments  and  buildings  may  usually 
be  built  directly  on  the  ground  near  the  site  where  they  are 
to  be  sunk. 

When  built  on  ways  the  caisson  sometimes  has  a  false  bottom 
fitted  to  it  to  reduce  the  depth  of  immersion,  and  a  sufficient 
height  of  crib  is  constructed,  preliminary  to  launching,  to 
insure  the  top  being  well  above  water-level.  After  launching 
and  towing  to  the  site  more  crib  is  added,  the  false  bottom 
removed  and  the  caisson  sunk  to  the  river  bed  by  placing 
concrete  in  the  crib. 

The  launching  ways  used  for   the  McKinley  bridge   over 


316  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 

the  Mississippi  River  at  St.  Louis,  Mo.,  consisted  of  a  number 
of  rows  of  piles  capped  with  timbers  running  at  right  angles  to 
the  river  and  on  a  slope  of  if  inches  per  foot.  Each  caisson  was 
built  on  shoes  extending  the  full  width  of  the  caisson,  the  long 
side  of  the  caisson  being  parallel  to  the  river,  and  each  shoe 
rested  on  a  cap  timber  on  which  it  slid  during  launching. 
These  shoes  were  spaced  about  6  feet  apart  and  were  so  made 
that  they  projected  down  over  the  sides  of  the  caps.  They 
were  bolted  to  the  latter  on  the  land  side  of  the  caisson.  The 
caisson  was  built  with  its  bottom  in  a  horizontal  position  by 
using  wedges  between  the  caisson  and  the  shoes.  The  launch- 
ing was  started  by  simultaneously  sawing  through  the  shoes 
below  the  bolts,  which  thus  allowed  the  caisson  to  slide  into  the 
water. 

Fig.  1 05  a  shows  the  caisson  for  one  of  the  piers  of  the  Van- 
couver bridge,  Vancouver,  Wash.,  as  it  was  being  built  on  the 
launching  ways.  The  general  scheme  was  about  the  same  as 
for  the  McKinley  bridge  caissons. 

Where  built  on  floats,  either  one  or  two  pontoons  may  be  used. 
Fig.  10  §b  shows  one  of  the  caissons  of  the  Willamette  River 
bridge  of  the  Northern  Pacific  Railroad  as  it  was  being  built 
between  two  barges  or  pontoons.  The  caisson  was  held  be- 
tween the  barges  until  a  height  of  20  feet  had  been  built  up, 
when  long  screws  were  attached  and  the  caisson  lowered  into  the 
water.  Two  heavy  trusses,  one  at  each  end,  tied  the  barges 
together  to  prevent  any  unequal  motion  of  the  latter  by  the 
waves.  Another  caisson  for  the  same  bridge  was  erected  on 
two  pontoons,  and  after  building  to  a  sufficient  height  the 
pontoons  were  scuttled  by  filling  them  with  water,  after  which 
they  were  pulled  out  from  under  the  caisson. 

The  78  X  144-foot  caisson  of  the  Manhattan  bridge  was  built 
in  a  pontoon  or  float,  84  feet  wide  and  150  feet  long,  which  had 
vertical  sides  8  feet  high.  The  float  was  built  of  3-inch  planks 
bolted  to  vertical  and  horizontal  timbers.  It  was  built  in  two 
halves  separated  by  a  longitudinal  joint  along  the  center  line. 
Blocking  was  set  up  on  the  floor  timbers  and  on  this  the  caisson 
was  built,  thus  making  the  latter  accessible  from  below.  On 


FIG.  io5a. — Caisson  on>  Launching  Ways.     Vancouver  Bridge. 


FIG.  1056. — Cassion  Supported  between  Two  Barges.     Willamette  River  Bridge. 

(Facing  p.  316.) 


ART.  1 06  SINKING  THE   CAISSON  317 

completing  the  caisson  the  joint  between  the  two  halves  of  the 
float  was  unlocked  and  sand  dumped  through  the  shafts  of  the 
caisson  to  the  floor  of  the  float  to  sink  the  halves  of  the  latter, 
after  which  the  same  were  pulled  from  beneath  the  caisson. 

Fig.  105^  shows  one  part  of  the  4oXioo-foot  pontoon  of 
the  St.  Louis  Municipal  bridge  caissons  as  it  was  being  pulled 
from  beneath  the  caisson.  This  pontoon,  which  was  of  the 
same  type  as  that  described  above,  was  sunk  by  removing  plugs 
from  holes  in  the  bottom  of  the  pontoon. 

The  caissons  for  the  Passyunk  Ave.  bridge  piers  offer  a  good 
example  of  caissons  built  on  a  platform.  Sixteen  bearing  piles 
were  first  driven  in  two  longitudinal  rows  just  clear  of  the 
caisson  location.  These  were  capped,  and  from  these  cap 
timbers  four  equidistant,  transverse,  i4Xi6-inch  timbers  were 
suspended  by  pairs  of  ij-inch  rods,  16  feet  long,  threaded  the 
entire  length,  and  each  provided  with  two  nuts.  Each  trans- 
verse timber  was  held  by  means  of  a  steel  saddle  on  the 
under  side,  against  which  the  lower  nut  of  the  rod  bore  and  the 
other  nut  took  bearing  on  a  washer  on  top  of  the  pile  cap. 
The  transverse  timbers  were  first  screwed  up  tightly  against  the 
under  side  of  the  cap  timbers  and  on  these  the  caisson  was 
built.  After  building  the  cribs  to  a  height  of  about  26  feet 
the  caisson  and  transverse  timbers  were  gradually  lowered  by 
unscrewing  the  nuts  from  the  rods,  which  permitted  the  caisson 
to  float  in  its  exact  position. 

ART.  106.    SINKING  THE  CAISSON 

If  mud  covers  the  river  bottom  this  should  be  dredged  out 
before  placing  the  caisson  as  it  is  cheaper  to  remove  it  in  this 
manner  than  to  excavate  it  within  the  working  chamber. 
Great  care  must  be  exercised  in  grounding  the  caisson  to  place 
it  in  its  correct  position.  If  in  tidal  water,  this  may  be  done 
by  placing  concrete  in  the  crib  to  an  amount  which  will  just 
ground  the  caisson  at  low  tide.  Then,  by  means  of  tackles 
attached  to  clusters  of  piles  and  to  the  caisson  or  crib,  the 
structure  is  placed  in  its  true  position  at  high  tide  and  grounded 


318  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 

as  the  water-level  lowers.  Concrete  is  then  poured  into  the  crib 
to  an  amount  which  will  prevent  floating  when  the  tide  rises. 
Often,  where  the  caisson  is  slightly  out  of  position,  it  may  be 
floated  by  admitting  a  small  amount  of  air  into  the  working 
chamber.  As  soon  as  enough  concrete  has  been  placed  to  put 
on  air  pressure  safely  to  expel  the  water  from  the  working 
chamber,  men  enter  to  commence  sinking  operations. 

In  clay  the  excavation  may  usually  be  kept  some  distance 
below  the  cutting  edge,  which  offers  the  advantage  of  allowing 
more  head-room  for  the  men.  This  cannot  be  safely  done  in 
sand  as  the  water  is  very  sensitive  to  changes  of  pressure 
and  so  it  is  not  possible  to  raise  the  pressure  very  much  from 
that  corresponding  to  the  head  on  the  cutting  edge.  In  one 
of  the  caissons  of  the  Rulo  bridge  a  test  well  was  sunk  in  clay 
17  feet  below  the  cutting  edge  without  any  increase  in  the  air 
pressure,  but  when  a  4-foot  vein  of  gravel  was  struck  the 
pressure  had  to  be  increased  8  to  10  pounds  at  once. 

In  sinking  caissons  the  load  is  at  first  usually  carried  on  the 
cutting  edge,  but  as  the  caisson  gradually  sinks  more  of  the 
load  is  resisted  by  friction  on  the  sides  and  less  by  bearing  on 
the  cutting  edge.  Contrary  to  the  usual  custom,  in  the  case  of 
the  55  X  i8o-foot  caisson  of  the  New  Quebec  bridge,  the  details 
of  which  are  shown  in  Figs.  936,  c,  and  d,  and  which  for  the  most 
part  was  sunk  through  sand,  the  load  was  not  at  any  time 
supported  on  the  cutting  edge. 

l"  Owing  to  the  great  size  of  the  caisson,  extraordinary  pre- 
cautions were  considered  necessary  to  provide  against  any 
unequal  settlement,  or  any  twisting  or  other  movement  of  the 
caisson,  which  might  tend  to  open  up  the  joints  and  seams  and 
consequently  allow  air  to  escape.  On  this  account  it  was 
decided  that  the  ordinary  method  of  sinking,  where  all  the  load 
is  carred  on  the  cutting  edge,  would  not  allow  the  movements 
of  the  caisson  to  be  sufficiently  controlled  during  the  actual 
sinking.  The  rather  unusual  method  was  therefore  employed 
of  carrying  the  entire  load  on  the  bulkheads  and  the  roof,  and 
no  load  at  all  on  the  cutting  edge. 

1  Engineering  News,  vol.  68,  page  854,  Nov.  7,  1912. 


ART.  107       REMOVING   SPOIL   FROM   WORKING   CHAMBER  319 

"The  caisson  was  supported  on  40  sand  jacks,  about  25  posts 
of  12 X i2-inch  yellow  pine,  and  54  sets  of  blocking.  The 
jacks  and  posts  bore  directly  against  the  roof,  while  the  blocking 
was  piled  under  the  bulkheads.  When  ready  for  a  drop  the 
blocking  and  posts  were  first  removed  by  washing  the  sand  from 
under  them  with  a  water-jet;  then  the  whole  caisson  was 
lowered  by  operating  all  the  sand  jacks  simultaneously.  The 
sand  jacks  were  of  simple  construction,  each  one  consisting  of  a 
29-inch  steel  cylinder  closed  at  the  bottom,,  having  near  the 
bottom  two  3 -inch  holes  with  a  sliding  cover,  and  a  plunger 
consisting  of  a  single  piece  of  timber  fitting  easily  into  the 
cylinder.  The  cylinder  was  filled  two-thirds  full  of  sand,  the 
plunger  inserted,  and  its  upper  end  blocked  against  the  roof. 
The  operation  of  lowering  consisted  in  opening  the  lower  holes 
and  inserting  a  water-jet,  thus  washing  out  the  sand. 

"These  jacks  worked  admirably,  the  result  being  that  the 
caisson  was  sunk  absolutely  level  and  in  its  proper  location. 
Before  each  drop  a  trench  was  excavated  under  the  cutting 
edge  to  a  depth  of  2  or  3  feet,  and  filled  with  clay,  which  tended 
to  prevent  the  escape  of  the  air  and  also  acted  as  a  lubricant 
during  sinking.  This  scheme  was  followed  throughout  the  entire 
sinking  and  seemed  to  materially  facilitate  the  operation." 

Sinking  the  caisson  is  accomplished  by  excavating  the 
material  in  the  working  chamber  and  by  placing  concrete  in  the 
crib  to  weight  the  structure.  The  water-jet  is  sometimes 
employed  to  reduce  friction  on  the  sides. 

ART.  107.     REMOVING  SPOIL  FROM  WORKING  CHAMBER 

Various  devices  have  been  developed  for  removing  the  spoil 
from  the  air  chamber.  Where  the  material  is  sand  the  blow-out 
process  or  mud-and-sand  pump  is  ordinarily  employed;  where 
clay  is  encountered  it  is  usually  best  to  remove  it  with  buckets, 
using  some  simple  form  of  air-lock,  or  perhaps  the  clay  may  be 
mixed  with  water  and  the  sand-and-mud-pump  process  used. 
Boulders  must  be  removed  through  the  air-locks. 

BLOW-OUT  PROCESS. — The  blow-out  process  is  a  very  simple 


320  PNEUMATIC   CAISSONS  FOR  BRIDGES  CHAP.  IX 

affair,  the  principle  consisting  of  using  the  pressure  in  the  air 
chamber  to  drive  out  sand  or  mud  when  it  is  piled  around  the 
inlet  of  a  pipe  which  leads  from  the  working  chamber  to  the 
open  air.  The  diameter  of  the  pipe  is  usually  about  4  or  5 
inches,  the  top  being  fitted  with  an  elbow  to  throw  the  sand  in  a 
horizontal  direction,  while  the  lower  part  has  attached  to  it  a 
flexible  hose  of  large  diameter  with  a  valve.  To  blow  out 
the  sand  and  mud  it  is  only  necessary  to  heap  it  up  around  the 
mouth,  open  the  valve,  and  the  material  is  then  carried  out 
•with  a  high  velocity;  in  fact  the  velocity  is  so  great  that  the  pipe 
rapidly  wears  away.  At  the  Havre  de  Grace  bridge  the  elbow, 
which  was  of  chilled  iron,  4  inches  thick,  was  worn  through  in 
two  days.  Considerable  care  must  be  exercised  in  placing  the 
material  against  the  inlet  for  if  a  considerable  amount  of  air  is 
not  admitted  with  the  sand  and  mud,  it  will  clog,  while  if  there 
is  too  much  air  admitted  it  is  a  waste.  It  has  been  found  ad- 
vantageous to  have  small  holes  in  the  pipe  above  the  inlet  as 
this  gives  more  uniform  action,  tending  to  draw  the  material  up 
instead  of  merely  driving  it  and  thus  lessening  the  amount  of 
air  entering  with  the  sand  and  mud.  Although  the  dry  blow- 
out is  a  very  rapid  and  satisfactory  means  of  removing  the 
spoil  from  the  working  chamber  it  has  some  disadvantages: 
First,  a  tendency  to  vary  the  pressure. in  the  working  chamber; 
and  second,  a  tendency  to  cause  rapid  wear  in  the  pipe  elbow  as 
noted  above.  The  lowering  of  the  pressure  due  to  the  air 
passing  up  through  the  pipe  causes  a  very  thick  fog,  making  it 
difficult  for  the  workmen  to  see.  It  is  also  apt  to  allow  the 
water  to  enter  from  the  outside.  On  the  other  hand,  if  the  air 
compressors  are  supplying  air  at  a  rate  sufficient  to  maintain  a 
constant  air  pressure  when  the  sand  is  being  blown  out,  on 
stopping  the  latter  operation  the  pressure  may  rise  to  a  point 
sufficient  to  cause  a  blow-out  under  the  cutting  edge,  which  is 
usually  followed  by  a  flooding  of  the  air  chamber.  Largely  on 
account  of  the  destructive  action  on  the  pipe,  and  for  the  added 
reasons  just  noted,  the  dry  blow-out  process  is  most  satisfactory 
when  the  pressure  in  the  working  chamber  is  fairly  low,  although 
a  head  of  at  least  20  feet  is  necessary.  This  process  is  said  to 


ART.  107       REMOVING   SPOIL   FROM   WORKING   CHAMBER 


32I 


have  been  used  first  by  WILLIAM  SOOYSMITH  in  1859  in  building 
bridge  piers  over  the  Savannah  River. 

SAND-AND-MUD  PUMP. — The  principle  involved  in  this  form 
of  excavator  is  that  of  the  induced  current,  where  a  quantity 
of  water  with  a  high  velocity  causes  a  reduction  of  pressure 
which  draws  the  mud  and  sand — well  mixed  with  water — into 
the  pipe.  Fig.  107  a  illustrates  the  form  used  on  the  Memphis 
bridge.  The  water  enters  at  the  side  under  a  high  pressure  and 
passes  up  through  the  small  annular  space,  at  which  point,  on 
account  of  the  high  velocity,  the  pressure  is  low.  The  lower 
part  of  the  pump  connects  with  a  pipe  or 
hose,  the  lower  end  of  which  rests  in  a  pool 
of  mud  or  sand  and  water.  On  account  of 
the  difference  of  pressure  at  the  two  ends  of 
this  pipe  the  mud  is  drawn  into  the  pump 
and  carried  upward  with  the  water,  through 
a  pipe  which  connects  with  the  top  of  the 
pump.  The  essential  difference  between 
this  form  of  excavator  and  the  blow-out 
process  is  that  in  the  former  the  water  is 
the  moving  force  doing  the  work  while  in 
the  latter  it  is  the  air  from  the  working 
chamber.  The  water  pressure  used  is 
ordinarily  about  80  pounds  per  square 
inch.  This  method  was  first  used  by  JAMES 
B.  EADS  in  the  caissons  of  the  St.  Louis 

FIG.  xo-ja. — Sand-and- 

arch  bridge.  Fig.  gia  illustrates  another  Mud  Pui^p.  Memphis 
form  of  the  sand-and-mud  pump. 

In  the  Williamsburgh  bridge,  New  York,  the  hose  was  ex  tended 
to  a  sort  of  sump  in  the  bottom  of  the  excavation  where  its 
open  end  was  placed  below  the  surface  of  the  water.  Gravel,  sand 
and  mud  were  constantly  fed  into  the  nozzle  by  a  laborer  who 
raked  it  up  and  prevented  clogging,  and  another  man  with  a 
f-inch  nozzle  played  a  5o-pound  water-jet  against  the  soil  to 
wash  it  into  the  sump. 

For  a  description  of  this  process  as  applied  to  open-caisson 
work  the  reader  is  referred  to  Art.  91.  In  some  caisson  work  at 

21 


Vertical    Section. 


Horizontal    Section. 


322  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 

Arran,  Switzerland,  instead  of  using  a  sump  a  horizontal  hopper 
was  employed,  the  discharge  pipe  leading  from  the  lowest 
point  in  the  hopper.  A  jet  of  water  from  a  small  pipe  was  con- 
stantly played  on  the  material  as  it  was  fed  into  the  hopper. 

REMOVING  MATERIAL  WITH  BUCKETS. — Clay  is  usually  more 
cheaply  removed  with  buckets  than  by  any  other  method.  Large 
rocks  must  be  blasted  to  pieces  and  removed  with  buckets.  As 
stated  in  Art.  103  where  a  form  of  lock  similar  to  the  Moran  or 
O'Rourke  lock  is  used,  the  bucket  may  be  taken  from  the  lock 
without  removing  it  from  the  hoisting  rope.  In  the  form  shown 
in  Fig.  1030,  instead  of  running  the  hoisting  rope  to  an  engine  on 
the  outside,  the  hoisting  is  done  by  compressed  air  from  the 
working  chamber  working  in  the  cylinder  shown  on  the  left.  In 
this  cylinder  runs  a  piston,  the  two  sets  of  sheaves  being  so 
arranged  that  one  stroke  of  the  piston  lifts  the  bucket  the  whole 
distance. 

A  novel  device,  called  the  water  column,  was  used  in  the 
caissons  of  the  Brooklyn  bridge  to  remove  the  material.  It 
consisted  of  an  open  shaft,  the  lower  part  extending  into  a  sump 
which  was  kept  full  of  water  and  the  shaft  itself  was  filled  with 
water  up  to  a  point  sufficient  to  balance  the  air  pressure  in 
the  caisson.  Workmen  pushed  the  spoil  under  the  shaft  and 
from  there  it  was  removed  by  dredging  with  an  orange-peel  or 
clam-shell  bucket. 

ART.  1 08.     CONCRETING  THE  AIR  CHAMBER 

When  rock  is  reached,  if  the  same  is  level,  it  is  only  necessary 
to  clean  off  all  loose  material  before  depositing  the  concrete. 
On  the  other  hand,  if  not  level,  some  preliminary  work  must  be 
done;  if  the  rock  has  a  uniform  slope  it  should  either  be  blasted 
down  to  a  level  surface  or  else  stepped,  unless  very  rough ;  although 
if  the  rock  surface  is  at  practically  the  same  elevation  all  around 
the  cutting  edge  of  the  caisson,  but  irregular  within,  little 
more  than  a  thorough  cleaning  will  be  necessary.  For  those 
caissons  founded  on  clay  or  hard-pan  a  level  surface  is  easily 
obtained. 


ART.  109  RATE   OF   SINKING  323 

Caisson  No.  10  of  the  Passyunk  Ave.  bridge  landed  on  rock 
which  had  a  slope  of  about  5  feet  in  the  length  of  the  caisson. 
As  soon  as  rock  on  the  high  side  was  reached,  the  cutting  edge 
on  the  low  side  was  blocked  with  6Xi2-inch  timbers,  6  feet 
apart,  after  which  excavation  under  the  cutting  edge  was 
carried  to  rock  and  extended  i|  feet  out  beyond  the  cutting 
edge.  This  excavation  was  then  filled  with  concrete. 

In  the  caissons  for  the  St.  Louis  Municipal  bridge  the  rock 
surface  was  irregular  but  no  attempt  was  made  to  level  it 
off  or  to  bring  the  caissons  to  bearing  throughout.  Where 
depressions  occurred  the  sand  was  removed  and  sacks  of  concrete 
were  deposited  on  the  rock  and  tamped  under  the  cutting  edge, 
after  which  concrete  was  placed  in  the  working  chamber  in  the 
usual  manner. 

The  concrete  for  filling  the  working  chamber  may  be  carried 
in  through  the  material  shafts  and  locks  by  means  of  buckets, 
or  special  arrangements  may  be  made,  by  placing  a  cone-shaped 
frame  above  the  lower  door,  by  which  a  yard  or  more  of  concrete 
may  be  dumped  into  the  lock  through  the  upper  door.  The 
latter  is  then  closed  and  air  admitted  to  the  lock  allowing  the 
lower  door  to  open  and  the  mass  of  concrete  to  fall  through  the 
shaft  to  the  working  chamber.  The  conical  frame  prevents  the 
concrete  from  remaining  in  the  lock  when  the  lower  door  is 
opened.  For  a  description  of  the  method  used  in  placing  the 
concrete  in  the  working  chamber  see  Art.  186. 

ART.  109.     RATE  or  SINKING 

The  rate  of  caisson  sinking  varies  greatly,  the  larger  the 
caisson  and  the  harder  the  material  sunk  through,  the  slower 
the  rate.  Sinking  operations  are  usually  carried  on  day  and 
night,  and  the  rate  of  sinking  will  vary  from  almost  nothing 
where  beds  of  boulders  are  encountered  to  as  much  as  3  feet 
a  day  where  clean  sand  is  met.  Most  engineers  keep  a  chart 
of  the  progress  of  the  work;  Fig.  109^,  which  illustrates  the 
progress  in  sinking  one  of  the  caissons  of  the  Kinzie  St.  draw- 
bridge, Chicago,  is  a  very  satisfactory  form  of  chart  to  use.  The 


324 


PNEUMATIC   CAISSONS   TOR   BRIDGES 


CHAP.  IX 


caisson  is  shown  in  Figs.  logb  and  c.  Instead  of  carrying  the 
whole  caisson  to  bedrock  the  cutting  edge  was  stopped  about 
half  way  down  and  wells  were  then  sunk  the  remainder  of  the 
distance. 

In  sinking  Pier  D  of  the  Memphis  bridge,  excluding  long 
delays,   an   average   rate   of    1.5    feet    per   day   of   24  hours 


Weight 
Caisson 

ill 

|o 

h 

i.0 

\ 

LJ   < 

r~1  l 

i 

1 

:-g1 

1 

3  J  c 

jC;;j 

:[""i;3                    Pos  tion  of  Caisson  and  Water  Level  . 

S^  S^(^*>0«:2^:a:^5SRiipS^2^inh"J> 

itSSSfcS^ojcucufu^l-^^S^Sl^SSlSi^a 

^ 

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29J380 

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110 


120 


FIG.   lopa.  —  Progress  of  Sinking  Caissons.     Kinzie  St.  Bridge,  Chicago. 


was  maintained  through  sand  and  only  0.31  foot  through 
clay,  while  for  Piers  2  and  5  of  the  Thebes  bridge  the  average 
rates  were  0.23  and  0.41  foot  respectively;  here  hard  gravel 
was  encountered. 


ART.    109 


RATE    OF    SINKING 


325 


nMq  — 

i 

1 

BS? 

fc' 

<$ 

(^ 

-1^ 

> 

I 

P 

1 

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326 


PNEUMATIC   CAISS.ONS   FOR  BRIDGES 


CHAP.  IX 


The  rate  per  day  of  sinking  the  St.  Louis  Municipal  bridge 
caissons  varied  from  an  average  of  0.68  foot  for  Pier  4  to 
1.95  feet  for  Pier  3,  with  1.28  feet  as  an  average  for  all  caissons. 
The  best  progress  in  one  day  was  5.17  feet,  while  the  best 
seven-day  run  was  34  feet  or  4.86  feet  per  day.  For  the  caissons 

of  the  McKinley  bridge, 
St.  Louis,  the  average  rate 
for  all  caissons  was  2  feet 
per  day,  with  a  maximum 
of  7.7  feet  in  one  day. 


Cerrte\  Line   of  Track 


FIG. 


JPIan.r 

logc. — (See  also  Fig.  1096.) 


ART.  no.     FRICTIONAL 
RESISTANCE 


Estimating  the  probable 
frictional  resistance  to  be 
met  with  in  sinking  cais- 
sons is  one  of  the  most 
difficult  features  involved 
in  the  design.  It  depends 

upon  numerous  factors  such  as  the  kind  of  material  pene- 
trated; the  material  composing  the  sides  of  caisson  and  crib ; 
depth  to  which  sunk;  whether  the  sides  of  the  caisson  are  ver- 
tical or  flared;  whether  or  not  the  water-jet  is  used;  and  the 
amount  of  air  leaking  under  the  cutting  edge. 

In  general,  the  frictional  resistance  per  square  foot  of  exposed 
surface  of  caisson  and  crib  will  seldom  be  less  than  250  nor  more 
than  800  pounds,  although  in  boulder-strewn  material  it  may  be 
as  much  as  1000  pounds.  Next  to  mud  and  silt,  sandy  soils 
offer  the  least  resistance,  especially  when  carrying  large  amounts 
of  water,  while  clay  will  offer  less  resistance  than  material  con- 
taining boulders.  With  uniform  soil  conditions  the  unit  fric- 
tion will  increase  with  the  depth;  for  instance,  at  the  McKinley 
bridge,  which  crosses  the  Mississippi  River  at  St.  Louis,  the 
friction  was  found  to  be  about  300  pounds  per  square  foot  of 
exposed  surface  at  40  feet,  and  600  pounds  at  a  penetration  of 
70  feet.  Anything  which  tends  to  loosen  the  soil  around  the 


ART.  no 


FRICTIONAL   RESISTANCE 


327 


sides  of  the  caisson  and  crib  will  decrease  the  friction,  at  least  for 
a  short  time;  escaping  air  has  about  the  same  effect  as  the  water- 
jet  in  lubricating  the  material.  Although  flaring  out  the  bottom 
of  the  caisson  tends  to  reduce  the  side  friction,  yet,  on  account  of 
possible  wedging  action  by  material  falling  into  the  open  space 
above  the  bottom,  and  further,  on  account  of  the  loss  of 
guidance,  pneumatic  caissons  are  now  practically  all  made  with 
vertical  outside  walls. 

Table  No.  noa  gives  values  for  the  skin  friction  when  the 
caissons  were  well  down  for  a  number  of  notable  structures. 
Table  No.  1 10  b,  taken  from  an  article  by  H.  L.  WILEY  in  Trans- 
actions American  Society  of  Civil  Engineers,  vol.  62,  page 
113,  ^March,  1909,  gives  values  of  friction  for  both  open  and 
pneumatic  caissons. 


TABLE  NO. 

SKIN  FRICTION  FOR  PNEUMATIC  CAISSONS  OF  BRIDGES 

(Expressed  in  Pounds  per  Square  Foot) 


Name  of  bridge 

Range  for 
separate 
piers 

Aver- 
age 

No. 
of 
piers 

Materials  penetrated  in  sinking 
caissons 

Belief  ontaine.   . 

600—700 

648 

4 

Fine    sand,    sand,    coarse    sand, 

WT"W 

f 

boulders. 

Blair  Crossing  

330-410 

38l 

4 

Fine   sand,    coarse    sand,    clay. 

Brooklyn  

600 

Cairo  
Havre  de  Grace  

622-932 
308-489 

750 
400 

10 

4 

Sand.                 \ 
Silt,  sand,  mud.  u 

McKinley  

600 

Memphis  

36^-837 

^84 

c 

Sand,  gravel,  mud,  clay,  sedi- 

O    O        O  1 

G  ^T- 

O 

ment,  very  tough  clay,  quick- 

sand. 

Miles  Glacier  

620 

Nebraska  City  

409-590 

525 

3 

New  Omaha  

472-673 

617 

5 

Sand,  gravel,  some  clay  to  bed- 

rock. 

Rulo 

-}  ?  i—  Q44 

614 

4 

River  sand,  coarse  sand,  rubbish, 

OO           V/T'T' 

T" 

clay,  gravel. 

Sioux  City  

314-535 

463 

4 

Fine  sand,  yellow  sand,  gravel, 

clay,  boulders. 

Williamsburg  

750 

General  average  for  nine  bridges,  554  pounds  per  square  inch. 


328 


PNEUMATIC   CAISSONS   FOR   BRIDGES 


CHAP.  IX 


TABLE  NO.  1106 


No. 

Type  of  caisson 

Method  of 
sinking 

Material  penetrated 

Skin 
fric- 
tion 

Depth 
below 
low 
water 
in  feet 

Area  of 
base  in 
square 
feet 

i 

2 
3 
4 
5 
6 
7 
8 
9 
10 
it 

12 

13 

14 

IS 
16 
I? 
18 
19 

20 
21 
22 
23 
24 
25 
26 
27 

28 

29 

30 
31 
32 
33 
34 

Cast  iron  

Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Open  excavation 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 
Pneumatic 

Gravel,  clay  
Sand,  clay  
Sand 

240 
250 
250 
285 
300 
325 
350 
375 
390 
450 
450 
450 
450 
450 
480 
500 
700 
205 
250 
275 
310 
350 
400 
400 
425 
450 
500 
525 
540 
600 
650 
650 
660 
900 

60 
75 
60 
140 
100 

60 
60 
55 
75 
30 
60 
60 
65 
75 
65 
60 
65 
40 
35 
60 
75 

100 

48 
95 
55 
68 
75 
60 
75 
75 
8c 
90 

101 

45 

125 
225 
125 

1000 

125 
125 

125 

IQO 
100 
I3OO 
700 
1200 
I3OO 
1500 
200 

125 

1300 

75 
800 
ISO 
2550 

I20O 

1925 

4500 
1300 
2700 
1800 

I20O 

1700 
1400 

2OOO 
1200 
2100 
1700 

Cast  iron  
Cast  iron  

Wrought  iron  
Cast  iron.  .  .  .  

Sand,  clay 

Sand,  clay,  gravel.  .  .  . 
Sand. 

Cast  iron  

Cast  iron  
Steel  construction.  .  . 
Cast  iron  
Timber  construction 
Steel  construction.  .  . 
Steel  construction.  .  . 
Steel  construction.  .  . 
Steel  construction.  .  . 
Iron  construction.  .  . 
Cast  iron  
Steel  construction.  .  . 
Masonry 

Silt  

Silt,  sand,  clay  
Silt,  mud,  clay  
Sand  
Silt,  clay  
Silt,  clay,  sand  
Mud,  sand  
Clay  
Sand,  gravel,  clay..  .  . 
Clay  

Clay  

Timber  construction 
Steel  construction.  .  . 
Timber  construction 
Timber  construction 
Timber  construction 
Timber  construction 
Timber  construction 
Steel  construction.  .  . 
Timber  
Iron  cylinder  
Timber  construction 
Timber  construction 
Timber  construction 
Timber  construction 
Timber  construction 
Timber  construction 

Clay  

Clay,  sand  

Silt,  sand,  mud  

Sand,  clay,  gravel.  .  .  . 
Sand,  clay,  boulders.  . 
Clay,  sand,  gravel.  .  . 
Sand,  gravel,  clay..  .  . 
Sand  boulders. 

Silt,  clay,  gravel  
Sand,  shale  
Sand  

Sand,  clay  
Sand,  gravel,  clay.  .  .  . 
Sand  
Sand,  boulders  
Silt,  sand,  clay  

In  sinking  the  Commercial  Cable  Building  caissons  the  fric- 
tional  resistance  varied  from  250  to  300  pounds  per  square  foot 
of  exposed  surface,  while  in  the  United  Fire  Insurance  Co. 
caissons  it  was  as  high  as  1000  pounds. 

The  highest  value  of  frictional  resistance  was  observed  in 
1910  while  sinking  the  concrete  caisson  for  the  pivot  pier  of  the 
reconstructed  swing  bridge  of  the  Grand  Trunk  Railway  at 
Black  Rock  Harbor  on  the  Niagara  River.  The  material  pene- 
trated was  a  very  sticky  red  clay.  The  concrete  open  caisson 
weighed  8700  tons  and  1084  tons  of  stone  and  pig  iron  were 


ART.  in    PHYSIOLOGICAL   EFFECTS    OF    COMPRESSED   AIR  329 

piled  on  top  of  it.     The  area  was  10235  square  feet,  thus  giv- 
ing a  frictional  resistance  of  1912  pounds  per  square  foot. 

ART.  in.     PHYSIOLOGICAL  EFFECTS  OF  COMPRESSED  AIR 

The  question  of  the  physiological  effects  on  the  human  system 
when  working  in  compressed  air  is  an  important  one  from  both 
the  humanitarian  and  financial  standpoints.  In  the  past  almost 
all  the  important  works  employing  compressed  air  have  levied 
a  heavy  toll  of  suffering  and  death  oh  the  'sand-hogs/  as 
caisson  workers  are  commonly  called.  For  instance,  on  the 
caisson  work  of  the  St.  Louis  bridge  there  were  119  cases  of 
so-called  caisson  disease,  with  14  deaths  from  the  same,  while 
on  the  Brooklyn  bridge  there  were  no  cases  of  illness,  with 
3  deaths.  These,  of  course,  were  early  examples;  at  the  pres- 
ent time,  owing  to  a  better  knowledge  of  the  disease,  the 
records  are  not  so  bad,  but  the  disease  still  claims  its  victims  in 
too  many  cases. 

No  harmful  effects  are  felt  on  entering  the  compressed  air, 
or  while  remaining  in  it;  only  during  decompression  or  after 
emerging  are  the  workmen  affected.  The  disease,  which  has 
been  proven  to  be  aeremia,  may  be  divided  into  two  classes: 
First,  that  in  which  the  attack  is  light;  and  second,  that  in  which 
it  is  severe.  The  first  form  is  characterized  by  very  severe 
pains,  chiefly  in  the  joints,  and  closely  resembles  rheumatism 
in  its  effects.  From  the  tendency  to  cause  its  victim  to  double 
up  in  agony  it  is  commonly  known  as  the  'bends/  When 
the  attack  is  very  severe  it  usually  paralyzes  its  victim  and  is 
commonly  fatal. 

SENSATIONS  FELT  ON  ENTERING  THE  AIR  CHAMBER. — On 
entering  the  air-lock  and  having  the  air  pressure  turned  on, 
some  of  the  sensations  felt  are  heat,  slight  giddiness  and  head- 
ache, pain  in  the  ears,  breathlessness,  inability  to  whisper- 
caused  by  the  resistance  of  the  compressed  air  to  the  finer 
muscular  movements  of  the  tongue — and  a  feeling  of  resistance 
to  movement  owing  to  the  density  of  the  air.  A  slight  dis- 
comfort is  usually  felt  in  maintaining  equilibrium  between 


33°  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 

the  air  pressure  inside  and  outside  the  body,  the  most  painful 
being  in  the  ears,  as  noted  above.  This  may  be  overcome  by 
closing  the  mouth  and  holding  the  nose,  and  at  the  same  time 
trying  to  expel  the  air  from  the  lungs;  such  action  makes  the 
pressure  in  the  tympanic  cavity  equal  to  the  outside  pressure 
by  means  of  the  Eustachian  tubes  which  run  from  the  back  of 
the  nasal  passages  to  the  cavity.  This  action  should  be  repeated 
from  time  to  time  and  as  long  as  the  pressure  continues  to  in- 
crease. Relief  may  also  be  secured  by  the  action  of  swallowing. 
A  cold  makes  the  feat  more  difficult  since  the  Eustachian  tubes 
are  then  somewhat  blocked. 

Owing  to  breathing  the  denser  air  with  its  increased  amount  of 
oxygen,  as  soon  as  equilibrium  obtains  the  general  effect  is  some- 
what exhilarating  and  bracing.  To  quote  one  of  the  workmen  in 
the  Blackwall  tunnel  (England):  1UI  never  felt  happier  than 
when  I  was  in  the  compressed  air.  Always  happy,  and  on  the 
cheery  side.  Why,  laddie,  I  would  get  up  in  the  morning  feeling 
very  dour  and  queer,  and  just  go  into  the  workings  and  then 
whistle  (?)  and  sing  all  day  long." 

SENSATIONS  FELT  ON  LEAVING  AIR  CHAMBER. — On  leaving 
the  air  pressure  the  caissonier  feels  cold,  and  this  is  felt  most 
keenly  during  the  passage  through  the  air-lock,  being  due  to  the 
expansion  of  the  air  in  the  lock,  as  well  as  to  the  expansion  and 
liberation  of  gases  in  the  body.  To  counteract  the  effects  of 
this  cold  the  air-lock  should  be  warmed  and  the  men  given 
strong  hot  coffee  to  drink  on  emerging,  and  should  dress  warmly. 
Another  characteristic  of  decompression  is  a  dense  fog  which 
occurs  as  the  air  becomes  rarefied.  Another  sensation  often 
manifested  on  emerging  is  an  itching,  pricking  feeling  under  the 
skin  on  all  parts  of  the  body;  this  disappears  in  a  few  minutes. 
The  foregoing  are  the  sensations  always  felt;  if  the  person  is 
taken  with  caisson  illness  the  symptoms  may  be  manifold. 

1 "  Coming  out  again  (from  the  working  pressure)  it  was  not  so  bad, 
but  just  chilly;  bitter  chilly,  cold  as  charity.  The  pains  would  come  on 
afterward,  in  an  hour  or  so,  or  when  you  got  into  bed.  Bends  in  the 

1  Engineering  News,  vol.  51,  page  437,  May  5,  1904. 


ART.  in    PHYSIOLOGICAL   EFFECTS   OF   COMPRESSED  AIR  331 

back,  the  wrists  and  the  legs;  just  awful.  Men  would  turn  out  in  the 
middle  of  the  night  and  come  back  to  the  works  and  get  into  the  compressed 
air  again  in  the  medical  locks.  They  had  a  full  dose  for  a  start,  and  let 
the  pressure  drop  gradually.  Then  they  went  back  home  to  bed.  Do 
them  any  good?  Eh,  mon,  its  no  for  me  to  say.  They  thought  so,  but 
I  thought  it  was  only  humbug,  a  faith  dodge.  When  I  had  bends  I  just 
jumped  about  and  took  a  drap  qf  guid  whuskey — better  than  all  your 
doctor's  concoctions."  The  foregoing  graphic  description  of  the  'bends' 
and  treatment  for  it  indicates  the  attitude  of  the  average  'ground-hog.' 

1(1  The  symptoms  of  caisson  disease  have  been  quite  definitely  estab- 
lished. First  among  these  are  neuralgic  pains  of  an  intermittent  or 
paroxysmal  character,  and  of  varying  severity.  In  the  worst  instances 
these  pains,  or  cramps,  as  they  are  commonly  called — although  they  are 
rarely  accompanied  by  muscular  spasms — are  so  intense  as  to  completely 
unnerve  strong  men.  This  symptom  is  very  seldom  absent,  and  from  it 
comes  the  popular  name  of  'bends'  given  to  the  disease.  Another 
characteristic  symptom  which  is  always  exhibited  is  a  profuse  cold  per- 
spiration. Another  symptom  which  is  of  frequent  occurrence,  but  which 
is  not  always  exhibited,  is  pain  at  the  pit  of  the  stomach,  usually,  but  not 
always,  attended  by  vomiting.  In  about  50  percent  of  the  cases  observed, 
paralysis  has  been  a  characteristic  symptom.  The  degree  of  paralysis 
varies  from  slightly  impaired  sensation  or  numbness  in  the  extremities 
to  complete  loss  of  sensation  and  motion  in  the  affected  parts,  which  are 
most  frequently  the  legs  and  lower  part  of  the  body.  Finally  the  sufferer 
usually  exhibits  a  number  of  transient  symptoms,  which  have  their  origin 
in  the  brain;  these  are  headache,  dizziness,  double  vision,  incoherence  of 
speech,  and  sometimes  unconsciousness.  The  duration  of  these  symptoms 
varies  from  a  few  hours  to  several  weeks  in  case  of  paralysis.  In  fatal 
cases  congestion  of  the  brain  or  spinal  cord  always  exists.  A  very  notice- 
able fact  is  that  the  attack  of  the  disease  never  takes  place  while  the  sub- 
ject is  under  air  pressure,  but  always  occurs  while  he  is  emerging  from  the 
compressed  air  chamber  or  after  he  has  emerged." 

CAUSES  OF  CAISSON  DISEASE. — Various  theories  have  been 
advanced  from  time  to  time  relative  to  the  cause  of  caisson 
disease.  It  is  said  that  attention  was  first  called  to  caisson 
disease  at  about  the  middle  of  the  last  century  by  TRIGER  who 
applied  the  use  of  compressed  air  in  sinking  some  coal  shafts 
at  Chalons  on  the  banks  of  the  Loire.  2 "  HOPPE  SEYLER  (1857) 
and  THOMAS  SCHWANN  (1858)  in  Germany,  andBusQUOY  (1861) 

1  Engineering  News,  vol.  46,  page  157,  Sept;  5,  1901. 

2  Engineering  Record,  vol.  63,  page  362,  April  i,  1911. 


33  2  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 

in  France,  .  .  .  gave  the  first  correct  suggestion  as  to  the 
cause:  viz.,  that  it  was  due  to  the  setting  free  of  bubbles  of  gas 
in  the  blood.  Nitrogen  gas  is  dissolved,  according  to  the  law 
of  partial  pressures,  during  exposure  to  the  compressed  air,  and 
this  dissolved  gas  having  no  time  to  escape  through  the  lungs, 
if  the  pressure  be  suddenly  lowered,  bubbles  off  just  as  carbonic 
acid  escapes  from  aerated  water  when  a  bottle  is  uncorked." 

In  1871,  DR.  JAMINET,  the  physician  in  charge  of  the  com- 
pressed air  workers  at  the  St.  Louis  bridge,  became  convinced 
from  his  studies  that  the  disease  was  caused  by  too  rapid  a 
tissue  change  due  to  the  absorption  of  an  excess  of  oxygen. 

About  two  years  later,  DR.  A.  H.  SMITH,  the  surgeon  in 
charge  of  the  New  York  tower  caisson  of  the  Brooklyn  bridge, 
arrived  at  the  conclusion  that  the  ill  effects  developed  in  work- 
ing under  compressed  air  were  due  to  the  pressure  of  the  air  forc- 
ing the  blood  from  the  surface  of  the  body  to  the  center  and 
thereby  causing  internal  congestion. 

But  it  was  PAUL  BERT,  who,  by  his  remarkable  experiments, 
published  in  1878,  proved  the  true  cause  of  caisson  disease  to  be 
the  effervescence  of  gas  in  the  blood  and  tissue  juices.  Since 
then  such  authorities  as  PHILLOPON,  VON  SCHROTTER,  HELLER, 
MAGER,  HALDANE,  HILL,  SMITH,  MACLEOD,  GREENWOOD  and 
others,  have  checked  and  extended  BERT'S  experiments. 

The  gas  which  is  present  in  the  blood,  and  which  comes  out 
of  solution  if  the  pressure  is  too  rapidly  lowered,  is  mostly 
nitrogen,  for  if  the  chamber  is  properly  ventilated  there  will 
be  only  a  small  amount  of  carbonic  acid  gas  in  the  air,  while 
the  oxygen  content  dissolved  by  the  blood  is  taken  up  chemically 
by  the  hemoglobin,  as  demonstrated  by  DR.  HALDANE.  As 
stated  elsewhere  the  tissue  fluids,  chiefly  the  blood,  dissolve  the 
air  according  to  D ALTON'S  law  of  solution  of  gases  in  fluids; 
i.e.,  the  amount  of  gas  dissolved  in  a  fluid  is  proportional  to  the 
pressure  of  the  gas  surrounding  the  fluid.  Except  for  very 
high  pressures,  such  as  eight  or  ten  atmospheres — values  which 
will  never  attain  in  caisson  work — these  dissolved  gases  probably 
have  no  chemical  effect  on  the  system,  and  are  quite  harmless 
as  long  as  they  remain  in  solution.  For  high  pressures  the  dis- 


ART.  112  PREVENTION   OF   CAISSON  DISEASE  333 

solved  oxygen  seems  to  have  a  toxic  effect,  causing  a  fatal 
inflammation  of  the  lungs.  Experiments  have  shown  that  with 
a  pressure  of  ten  atmospheres  some  animals  will  die  in  as  short 
a  time  as  20  minutes. 

fc  However,  when  the  pressure  of  the  surrounding  air  is  lowered, 
the  dissolved  gases,  mostly  nitrogen,  are  thrown  out  of  solution 
in  the  form  of  bubbles.  If  the  lowering  of  the  pressure  is  done 
slowly  the  gases  are  thrown  out  of  the  blood  at  the  lungs  without 
developing  bubbles  of  any  appreciable  size.  But  if  the  pressure 
is  rapidly  lowered  the  gas  bubbles  stick,  owing  to  their  size, 
in  the  minute  blood  vessels  and  obstruct  the  flow  of  the  blood, 
often  causing  the  vessels  to  burst.  The  same  condition  ob- 
tains in  the  various  tissues  carrying  juices  saturated  with 
gas;  if  these  bubbles  develop  in  the  joints,  we  have  the 
'bends';  if  in  the  spinal  cord,  paralysis;  if  in  the  heart,  heart 
failure,  etc. 

ART.  112.     PREVENTION  OF  CAISSON  DISEASE 

If  the  cause  of  caisson  illness  is  a  mechanical  action  due  to 
the  development  of  bubbles  in  the  blood  and  fluid  tissues,  which 
in  turn  is  due  to  too  rapid  decompression,  then  manifestly  the 
cure  is  decompression  at  a  rate  slow  enough  to  avoid  this  phe- 
nomenon. The  length  of  time  will  depend  upon  the  amount  of 
gas  in  the  fluid  tissues  and  upon  the  physical  characteristics  of 
the  person  being  decompressed.  The  amount  of  gas  in  the  fluid 
tissues  will,  in  turn,  depend  upon  (i)  the  degree  of  pressure  in 
the  working  chamber  and  (2)  the  length  of  time  under  pressure. 
The  length  of  time  taken  to  saturate  the  body  fluids  at  any 
particular  pressure  will  vary  greatly,  depending  upon  the  fat- 
ness of  the  subject,  the  amount  of  bodily  work  done,  heat  and 
moisture  present,  etc.  From  experiments  DR.  HALDANE  con- 
cluded that  in  certain  parts  of  the  body  where  the  circulation 
is  rapid  and  the  number  of  blood  vessels  high  the  tissue  juices 
will  become  50  percent  saturated  in  5  minutes,  with  complete 
saturation  in  40  minutes;  while  other  parts,  lacking  a  copious 
supply  of  blood,  will  require  75  minutes  for  50  percent  saturation 


334  PNEUMATIC   CAISSONS  FOR  BRIDGES  CHAP.  IX 

and  about  4  hours  for  90  percent  saturation.  Experiments  show 
that  the  fatty  tissues  absorb  about  five  times'  as  much  gas  as 
does  the  blood  and  the  rate  of  absorption  is  much  slower;  the 
rate  of  desaturation  will  be  correspondingly  slow.  For  this 
reason  men  inclined  toward  fatness  should  never  be  employed 
for  compressed-air  work.  The  better  the  circulation  of  the 
blood  the  more  quickly  and  easily  will  the  gases  be  thrown  out 
of  the  system;  for  this  reason  only  men  in  good  physical  condi- 
tion should  be  employed.  Old  men,  or  those  who  have  abused 
themselves  by  excessive  drinking  or  other  dissipation,  should 
never  be  allowed  in  the  working  chamber. 

Authorities  differ  as  to  the  time  that  should  be  allowed  for 
decompression,  but  all  agree  that  the  usual  time  given  is  too 
short.  Some  urge  a  uniform  rate  of  decompression,  while 
others  prefer  stage  decompression,  that  is,  at  first  a  rapid  decom- 
pression to  a  certain  pressure,  followed  by  slower  decompression. 

Seldom  is  more  than  15  or  1 8  minutes  given  to  decompression; 
the  reason  for  this  is  that  the  air-lock  is  small  and  as  a  conse- 
quence the  men  must  maintain  cramped  positions  in  the  same. 
Moreover,  the  lock  is  usually  cold  and  filled  with  fog,  due  to 
the  decreasing  pressure.  Properly,  the  lock  should  be  large 
enough  to  allow  the  men  some  freedom  of  motion  and  it  should 
be  ventilated  with  warm  dry  air.  The  French  law,  enacted  in 
1908,  prescribes  that  for  a  head  of  water  up  to  65.6  feet  not  less 
than  21.2  cubic  feet  of  air  shall  be  provided  for  each  man  in  the 
lock,  and  for  depths  above  this  not  less  than  24.7  cubic  feet. 
During  decompression  the  men  should  constantly  move  about 
and  massage  their  various  joints,  as  this  has  been  found  to  assist 
materially  in  ridding  the  system  of  the  gases. 

MACLEOD  suggests  the  following  time  for  decompression  as 
being  safe: 

Gage  Pressure  Length  of  Shift  Decompression  Period 

15  to  30  4  hours  £  to  i  hour 

45  to  60  4  hours  i^  to  2  hours 

In  Germany,  VON  SCHROTTER,  HELLER  and  MAGER,  in  1900, 
published  a  work  in  which  they  laid  down  the  principle  that  a 


ART.  112 


PREVENTION    OF    CAISSON   DISEASE 


335 


uniform  decompression  at  the  rate  of  two  minutes  per  o.i  atmos- 
phere, or  20  minutes  per  atmosphere,  was  safe. 

The  law  of  New  York  State  (1913)  governing  the  time  of 
decompression  for  pneumatic  caisson  work  for  bridges  and  build- 
ings is  as  follows: 

Gage  pressure  in  pounds  10     15      20     25     30    36    40     50 

Time  of  decompression  in  minutes        i       2       5      10     12     15     20     25 

The  time  of  work  in  caissons,  given  by  this  law,  is  as  follows: 


Gage  pressure 

0-21 

22-30 

31-35 

36-40 

4i-4S 

45-So 

Time  per  day  in 
caisson. 
No.  of  shifts  
Length  of  shift  .    .  . 

8  hrs. 
2  (minimum) 

6  hrs. 

2 

3  hrs. 

4  hrs. 

2 

2  hrs. 

3  hrs. 

2  (min.) 
i^  hrs. 

2  hrs. 

2  (min.) 
i  hr. 

ii  hrs. 

2 

f  hr 

Minimum  time  be- 
tween shifts. 

30  consecutive 
minutes 

i  hr. 

2  hrs. 

(max.) 
3  hrs. 

(max.) 
4  hrs. 

5  hrs. 

The  theory  upon  which  stage  compression  is  based  is  that  the 
gas  in  the  blood  will  not  effervesce  until  a  marked  diminution  of 
pressure  obtains,  and  as,  to  the  point  of  effervescence,  the  gases 
are  discharged  at  a  rate  varying  with  some  function  of  the 
change  of  pressure,  manifestly  the  more  rapid  the  lowering  of 
pressure  the  more  quickly  will  the  blood  vessels  be  freed  of  the 
gases  contained  therein.  Since  almost  no  cases  of  aeremia  are 
caused  by  rapid  decompression  from  about  19  pounds  gage  pres- 
sure, it  seems  reasonable  to  assume  that  the  pressure  in  the  air- 
lock may  be  reduced  that  amount  in  about  three  minutes;  from 
this  point  the  pressure  must  be  lowered  quite  slowly  and  should 
correspond  to  the  natural  rate  of  desaturation  of  the  fluid 
tissues  at  that  difference  of  pressure.  When  the  gage  pressure 
reaches  about  19  pounds,  the  remainder  of  the  decompression 
may  be  done  quickly,  for,  as  stated  above,  it  appears  that  the 
average  person  can  safely  stand  that  difference  of  pressure.  The 
fundamental  idea  upon  which  stage  decompression  is  based  is 
correct,  but  as  there  is  but  little  experimental  data  and  less 
precedent  to  guide  one,  it  has  not  yet  become  general. 

Apart  from  the  matter  of  slow  decompression,  other  precau- 


33 6  PNEUMATIC   CAISSONS   FOR  BRIDGES  CHAP.  IX 

tions,  if  taken,  will  do  much  to  lessen  the  occurrence  of  caisson 
disease.  Anything  which  tends  to  lower  the  vital  resistance  of 
the  human  system  tends  to  promote  caisson  illness.  For  this 
reason  the  physical  conditions  under  which  the  men  work  should 
be  as  good  as  it  is  possible  to  make  them:  There  should  be 
furnished  plenty  of  fresh  air;  electric  lighting  rather  than  gas  or 
candle  lighting  should  always  be  employed,  as  the  latter  tends 
to  vitiate  the  air;  the  air  should  be  kept  at  as  reasonable  a 
temperature  as  possible,  which  means  that  it  should  be  cooled 
during  the  summer  time,  as  compression  raises  its  temperature. 
At  present  this  is  done  in  practically  all  work,  either  by  spraying 
the  compressed  air  as  it  enters  the  working  chamber,  or  else  by 
passing  it  through  a  coil  of  pipes  externally  cooled. 

lult  is  well  known  that,  in  a  confined  atmosphere,  man 
sooner  or  later  suffers  from  the  accumulation  of  poisonous 
gases.  The  criterion  of  this  pollution  of  the  atmosphere  is  the 
amount  of  carbonic  acid  (CC^)  found  present.  When  the  per- 
centage of  C02  in  the  air  rises  above  o.i  percent,  evil  effects  are 
common.  It  should  be  clearly  understood  that  these  evil 
effects  are  not  due  to  the  carbonic  acid  itself,  but  to  some  other 
toxic  property  which  the  CC>2  content  seems  to  run  parallel 
with,  and  is,  therefore,  a  measure  of  it.  Now  under  pressure 
it  is  evident  that  such  a  gas  will  be  still  more  dangerous.  As  a 
matter  of  fact,  E.  H.  SNELL  reports  that  an  'increase  of  CO2 
from  0.04  percent  to  o.i  percent  at  30  pounds  pressure  is  the 
forerunner  of  much  illness.'  He  found  that  by  free  ventilation 
of  the  caisson,  so  as  to  remove  this  C02,  the  illness  dropped  from 
seven  cases  a  day  to  one  case  in  two  days.  .  .  .  Ventilation  is  a 
matter  which  should  be  carefully  provided  for,  since  otherwise 
the  C02  and  other  poisonous  constituents  of  polluted  air  will 
have  their  usual  depressing  effects  on  the  workmen  and  render 
them  more  prone  to  suffer  from  decompression  symptoms." 

Especially  when  sinking  through  foul  material  should  care  be 
exercised  in  keeping  the  air  pure.  T  K.  THOMSON  reports  that 
when  sinking  through  the  foul  bottom  of  the  Harlem  River  the 

1  Cause,  Treatment  and  Prevention  of  the  Bends,  by  J.  J.  R.  MACLEOD, 
Journ.  Assoc.  Eng.  Soc.,  vol.  39,  page  301,  Nov.,  1907. 


ART.  112  PREVENTION   OF   CAISSON  DISEASE  337 

men  suffered  much  from  the  bends,  but  when  sinking  through 
the  clay  below  this,  even  though  under  a  much  greater  pressure, 
very  little  trouble  occurred.  It  is  also  noticed  that  a  greater 
amount  of  sickness  is  apt  to  occur  during  concreting  than  at 
other  times,  this  being  due  to  the  decrease  in  the  leakage  of 
the  air,  or  inadequate  ventilation. 

CURE  FOR  CAISSON  DISEASE. — The  best  and  about  the  only 
cure  for  caisson  disease  is  recompression  with  slow  decompres- 
sion. If  the  patient  can  be  put  into  the  air  before  the  gas  bub- 
bles have  had  a  chance  to  tear  the  blood  vessels  and  fluid  tissues 
a  cure  can  usually  be  effected,  but  otherwise  not.  For  this 
reason,  a  hospital  air-lock,  large  and  well  ventilated,  should 
always  be  maintained  in  readiness  and  the  men  should  be 
housed  near  by,  so  that  in  case  of  delayed  attacks  they  may 
be  immediately  recompressed. 


CHAPTER  X 

PNEUMATIC  CAISSONS  FOR  BUILDINGS 

ART.  113.     GENERAL  DEVELOPMENT 

The  application  of  the  pneumatic  caisson  to  building  founda- 
tions has  been  restricted  very  largely  to  the  tall  buildings  or 
'  skyscrapers7  of  New  York  City.  Two  conditions  occur  there 
which  require  this  form  of  foundation:  First,  the  necessity  for 
carrying  the  column  loads  to  bedrock;  and  second,  the  presence 
of  quicksand  over  the  rock.  Both  the  height  of  the  buildings 
and  the  magnitude  of  the  column  loads  make  it  imperative  to 
found  the  piers  on  a  very  hard  and  unyielding  stratum,  prefer- 
ably bedrock,  since  any  irregular  settlement  is  exceedingly 
dangerous  and  difficult  to  remedy  in  tall  buildings.  The  pres- 
ence of  quicksand  makes  sinking  to  bedrock  very  difficult  by 
other  methods  than  that  of  the  pneumatic  caisson,  due  to  the 
tendency  of  the  material  to  flow  into  the  excavation;  while  it  is 
especially  dangerous  in  the  lower  part  of  Manhattan  Island, 
due  to  the  liability  of  undermining  adjacent  building  founda- 
tions, many  of  which  rest  on  shallow  foundations.  The  only 
disadvantage  of  the  pneumatic  method  is  its  high  cost,  but 
this  is  fully  justified  where  the  security  of  very  expensive 
buildings  is  at  stake. 

In  its  details,  the  caisson  for  a  building  does  not  differ  mate- 
rially, except  in  the  matter  of  size,  from  the  bridge  caisson.  It 
is  customary  in  most  cases  to  use  separate  piers  for  all  the  inte- 
rior columns,  these  being  circular  or  square  in  plan;  but  special 
conditions,  such  as  the  close  spacing  of  two  or  more  columns,  or 
lack  of  clearance,  sometimes  makes  it  necessary  to  use  one  pier 
for  two  or  more  columns.  Where  the  grade  of  the  cellar  floor 
is  below  the  ground-water  line  the  wall  piers  often  serve  two 
functions:  First,  that  of  carrying  the  wall-column  loads  to  rock; 

338 


ART.  148  REINFORCED   ARCH   ABUTMENTS  451 

thus  making  the  beam  spacing  8  feet  center  to  center,  the  shaft 
spacing  longitudinally  being  16  feet  center  to  center. 

For  valuable  material  on  the  design  and  costs  of  various  types 
of  abutments  see  a  paper  by  J.  H.  PRIOR  in  Proceedings  of  Amer- 
ican Railway  Engineering  Association  (1912),  vol.  13,  page 
1085,  as  well  as  an  article  by  W.  M.  TORRANCE  on  The  Design 
of  High  Abutments,  in  Engineering  News,  vol.  55,  page  36, 
Jan.  n,  1906. 


CHAPTER  XV 

SPREAD  FOUNDATIONS 

ART.  149.     GENERAL  CONSIDERATIONS 

Foundations  for  buildings,  where  bedrock  is  some  distance 
below  the  surface,  are  of  three  general  types :  First,  those  carried 
deep  to  rock  or  hard-pan;  second,  those  in  which  piles  are  used; 
and  third,  those  spread  over  a  given  surface.  The  first  type  is 
widely  used  for  heavy  buildings  where  the  material  overlying  the 
rock  is  soft,  and  is  exemplified  in  the  pneumatic-caisson  process 
described  in  Chap.  X,  and  in  the  open- well  process  described 
in  Chap.  XI.  Although  the  most  expensive  type  of  founda- 
tion, it  offers  the  advantage  of  an  absolutely  unyielding  support 
for  the  buildings.  The  subject  of  bearing  piles  is  treated  in 
Chaps.  I  to  V  inclusive. 

The  object  of  the  shallow  type  of  foundation  is  to  spread  the 
load  over  a  considerable  horizontal  area  near  the  surface  of  the 
ground;  that  of  pile  foundations  to  distribute  the  load  over  a 
considerable  vertical  area — the  circumferential  surface  of  the 
piles — as  well  as  carrying  some  of  it  to  the  horizontal  stratum  at 
the  feet  of  the  piles;  while  the  deep  foundation  distributes  the 
load  over  a  relatively  small  area  on  the  rock  or  hard-pan.  Where 
rock  is  present  near  the  surface  there  is  no  foundation  problem, 
it  being  necessary  only  to  level  off  the  rock  with  a  layer 
of  concrete  and  place  the  columns  or  walls  directly  upon  it, 
although  a  spread  footing  may  be  used  where  the  foundation 
loads  are  very  heavy. 

In  many  localities  the  most  common  type  for  light  buildings 
is  the  shallow  foundation,  and  in  modern  development  it  is 
being  used  to  a  considerable  extent  for  heavy  structures.  In 
its  original  and  simplest  form  the  shallow  foundation  consists 
of  a  wide  concrete  or  masonry  footing  with  its  maximum  area  at 

452 


ART.  150  EARLY  TYPES  OF  FOOTINGS  453 

the  base  and  stepped  off  to  decrease  in  horizontal  area  toward  the 
top,  the  latter  being  of  sufficient  size  to  form  a  seat  for  the  wall 
or  column  base.  Although  this  makes  a  satisfactory  footing  for 
small  loads  it  is  not  well  adapted  to  heavy  loads  owing  to  the 
depth  required  to  get  the  necessary  spread  of  base.  Other  forms 
of  shallow  foundations  have  been  developed,  such  as  the 
wooden  grillage,  the  inverted  arch,  the  steel  I-beam 
grillage,  and  the  reinforced-concrete  spread  footing,  all  of 
which  require  less  depth. 

The  shallow  type  of  foundation  is  relatively  inexpensive, 
and  easily  and  quickly  constructed,  but  it  possesses  the  disad- 
vantage of  failing  to  furnish  a  rigid  and  unyielding  support  for 
the  building.  Where  founded  on  compact  sand  the  settlement 
will  be  slight,  seldom  more  than  J  inch,  but  where  founded  on 
a  material  like  the  Chicago  clay  the  settlement  may  in  time 
amount  to  2  feet  or  more.  Hence  heavy  buildings  resting  on 
shallow  foundations  are  built  to  allow  for  a  certain  amount  of 
settlement,  or  else  the  foundations  are  so  constructed  that 
powerful  hydraulic  jacks  can  be  used  to  raise  the  building  to 
permit  shimming  up.  Uniform  settlement  causes  but  little 
trouble  and  can  be  easily  taken  care  of;  but  unequal  settlement 
causes  the  walls  to  crack.  The  most  satisfactory  method  of 
guarding  against  unequal  settlement  was  early  found  to  be  the 
use  of  independent  footings  for  the  columns,  the  area  of  the  base 
of  each  footing  being  so  proportioned  that  the  unit-pressure  is 
the  same  under  all  footings. 

ART.  150.    EARLY  TYPES  OF  FOOTINGS 

MASONRY  FOOTINGS. — This  type,  which  was  one  of  the 
earliest,  is  still  the  standard  for  light  loads.  It  may  be  built  of 
concrete,  brick  masonry,  or  stone  masonry,  the  first  being  the 
most  widely  used  at  present.  In  designing  the  footing  the  area 
of  base  is  found  by  dividing  the  wall  load  by  the  safe  bearing 
power  of  the  soil  as  given  in  Art.  179.  To  safeguard  the 
masonry  against  crushing  the  compressive  unit-stress  on  any 
horizontal  section  should  not  exceed  the  values  given  in  Table 


454  SPREAD   FOUNDATIONS  CHAP.  XV 

1500.     The  top  of  the  footing  is  made  a  little  larger  than  the 
column  base  or  wall. 

Having  determined  the  top  and  bottom  areas  of  the  footing 
the  next  step  is  to  design  the  offsets,  which  fix  the  depth  of  the 
footing.  As  usually  designed  these  offsets  are  assumed  to  act 
as  free  cantilevers,  and  so  the  allowable  offset  of  any  section 
will  depend  upon  :  First,  the  pressure  on  the  under  side;  second, 
the  transverse  strength  of  the  masonry;  and  third,  the  thick- 
ness of  the  course.  The  center  of  gravity  of  the  base  should 
coincide  with  the  axis  of  the  load,  otherwise  additional 
stresses  will  develop. 

TABLE  1500 

Safe  corn- 

Character  of  masonry  pression,  Ibs. 

per  sq.  inch 
Common  brick,  hard  burned  (portland  cement  mortar)  ....  200 

Common  brick,  ordinary  (portland  cement  mortar)  ........  175 

Rubble  masonry,  uncoursed  (portland  cement  mortar)  .....  140 

Rubble  masonry,  coursed  (portland  cement  mortar)  .......  200 

Portland  cement  concrete,  1-2-4  mixture  .................  450 

Portland  cement  concrete,  1-2^-5  mixture  ................  350 

Portland  cement  concrete,  1-3-6  mixture  .................  250 

Considering  the  case  of  a  footing  for  the  wall  of  a  building, 
let  p  denote  the  unit-pressure  in  pounds  per  square  foot  on  the 
bottom  of  the  course  in  question;  R,  the  modulus  of  rupture  of 
the  masonry;  /,  the  factor  of  safety  used;  t,  the  thickness  of  the 
course  in  inches;  and  0,  the  allowable  offset  of  the  course  in 
inches.  The  following  formula  is  then  obtained, 


A  factor  of  safety  of  about  six  will  usually  be  advisable.  In 
designing  masonry  footings  for  columns  the  method  given  in 
Art.  158  is  recommended,  although  the  above  formula  may  give 
sufficient  precision. 

OTHER  EARLY  TYPES  OF  FOOTINGS.  —  Owing  to  its  lack  of 
transverse  strength  masonry  is  ill-adapted  to  take  loads  which 
cause  flexural  stresses  of  any  magnitude.  For  this  reason  vari- 


ART.  150 


EARLY  TYPES   OF  FOOTINGS 


455 


ous  substitutes  have  been  adopted,  the  idea  being  to  use  some 
material  having  considerable  transverse  strength  in  order  to 
reduce  the  necessary  depth. 

Among  the  early  types  was  the  timber  grillage.  This  con- 
sists of  two  or  more  layers  of  heavy  timbers,  each  layer  being 
placed  at  right  angles  to  the  one  above  and  below,  the  top  and 
bottom  being  often  sheathed  with  a  layer  of  planking.  The 

various  courses  are  well  tied 
together  with  drift  bolts. 
Examples  of  such  grillages 
have  been  dug  up  after  being 
buried  from  50  to  100  years 
and  where  below  ground- 
water  level  have  been  found 
to  be  in  a  perfect  state  of 


-Concrete  Piles- 


Average 


Ground  Line 


<-4^"Sj. 

-> 

- 

<-- 

j'Ocf.- 

->               \ 

«- 

.._, 

-9'Oct 

^ 

-K'Oct 

'    r 

r     y 

ENG.NE.WS 


FIG.   i.^oa. — A  Typical  Masonry  Footing. 


FIG.   1506. — Spread  Footing  of 
Timber  Column. 


preservation.  The  high  price  of  timber,  together  with  its  rela- 
tively low  transverse  strength  and  the  uncertainty  of  the  future 
ground-water  level,  makes  timber  an  undesirable  material  for 
use  in  permanent  foundations.  For  temporary  structures,  such 
as  exposition  buildings,  it  is  still  used  to  some  extent.  Fig. 
1 50  b  and  Fig.  150^  show  the  details  of  such  a  grillage  when 
used  under  columns. 

Another  type,  which  was  employed  in  some  of  the  early 


456 


SPREAD  FOUNDATIONS 


CHAP.  XV 


heavy  Chicago  buildings,  consisted  of  a  thick  concrete  platform 
continuous  over  the  whole  area  of  the  building  site,  forming  a 


c -  -  4  x  jy/,^  of  birders 
supported.  Undera// 
O/rders  these  P/eces  run 
'down  to  Footing. 

I k" Bolt 


FIG.   isoc. — Braced  Column  Footing. 


FIG.   150^. — Footing  of  Inverted  Masonry  Arch,  Drexel  Building,  Philadelphia,  Pa. 

deep  monolithic  slab  at  the  cellar-floor  level  and  on  which  the 
columns  and  walls  rested.     The  effect  of  variation  in  the  magni- 


ART.  151          MODERN   TYPES   OF   SPREAD   FOUNDATIONS  457 

tude  of  the  concentrated  loads  was  to  crack  the  concrete  bed  into 
a  number  of  independent  footings  and  this  was  naturally  fol- 
lowed by  great  irregularity  in  the  settlement  of  various  parts  of 
the  building.  Hence  this  form  of  footing  was  never  entirely 
satisfactory. 

Another  early  type  of  spread  foundation  was  that  of  the 
inverted  masonry  arch  which  was  first  used  in  the  Drexel 
Building,  Philadelphia,  Pa.,  built  in  1893,  the  details  of  which 
are  shown  in  Fig.  150  d.  Another  notable  example  of  the  use  of 
this  type  was  in  the  World  Building,  New  York  City.  Both 
of  these  structures  were  among  the  early  examples  of  the  modern 
steel  office  building.  In  the  Drexel  Building  the  arches  were 
made  of  brick,  which  distributed  the  column  loads  through  con- 
tinuous lines  of  concrete  bases  in  the  column  rows,  on  the  soil 
below.  Although  the  brick  masonry  arch  is  no  longer  used,  the 
principle  is  still  employed  in  the  reinforced-concrete  arch  footing 
described  in  Art.  159. 

ART.  151.     MODERN  TYPES  OF  SPREAD  FOUNDATIONS 

The  two  modern  types  of  spread  foundations  are  the  steel 
I-beam  grillage  and  the  reinforced-concrete  spread  footing. 
The  steel  I-beam  grillage  dates  back  to  the  types  described  in 
the  preceding  article;  but  since  it  is  still  a  standard  type  it  is 
here  described.  The  conditions  surrounding  its  development 
are  as  follows :  In  the  business  district  of  Chicago  the  soil  con- 
ditions are  peculiar,  made  ground  extending  to  a  depth  of  about 
14  feet  below  street  grade  while  below  that  occurs  a  stratum  of 
hard  stiff  clay  6  to  12  feet  thick.  Below  this  the  clay,  while 
having  the  same  general  characteristics  as  that  above,  becomes 
softer  and  remains  so  to  a  depth  of  75  feet  or  more.  The  upper 
stiff  clay  makes  a  first-class  foundation  bed,  but  the  softer  clay 
below  offers  little  supporting  power. 

After  the  great  Chicago  fire  most  of  the  new  buildings  were 
founded  on  masonry  footings  which  rested  on  this  hard  clay 
stratum.  Owing  to  the  rapid  increase  in  the  size  and  weight 
of  buildings  it  became  necessary  to  increase  the  area  of  the  base 


SPREAD   FOUNDATIONS  CHAP.  XV 

of  footings  and  this  in  turn  compelled  the  use  of  deeper  footings. 
As  the  bearing  power  of  the  soft  clay  below  the  hard  stratum  was 
small  the  only  practicable  method  of  obtaining  this  greater 
depth  was  to  extend  the  footing  up  into  the  cellar;  and  thus  the 
cellars  soon  became  filled  with  pyramids  of  masonry,  robbing 
them  of  valuable  space.  This,  together  with  the  fact  that  the 
masonry  footings,  on  account  of  their  large  mass,  formed  too 
large  a  proportion  of  the  total  load  and  were  expensive,  started 
the  search  for  a  better  type  of  footing  for  heavy  loads.  The 
type  thus  developed  consisted  of  crossed  layers  of  old  steel 
rails,  which  were  soon  superseded  by  steel  I-beams,  both 
shapes  being  thoroughly  embedded  in  concrete  as  a  protec- 
tion against  rust. 

Probably  the  first  building  in  America  to  be  built  on  a  steel 
grillage  foundation  was  the  Montauk  Block,  Chicago,  built  in 
1878,  and  designed  by  BURNHAM  and  ROOT,  architects.  The 
ordinary  masonry  footing  was  used  for  a  part  of  the  building,  but 
to  obtain  space  for  the  boiler  a  grillage  of  steel  rails  embedded  in 
concrete  was  used  in  one  part  of  the  cellar.  Soon  after  this  date 
the  price  of  steel  I-beams  dropped  sufficiently  to  make  them 
available  for  this  purpose.  On  account  of  their  larger  section 
modulus  for  any  given  weight  per  foot,  they  are  much  more 
economical  than  rails,  and  in  a  short  time  I-beams  were  adopted 
exclusively.  For  very  heavy  loads  built-up  girders  are  often 
used  in  place  of  I-beams. 

ART.  152.     CONSTRUCTION  or  I-BEAM  GRILLAGES 

In  the  construction  of  I-beam  grillages  two  or  more  tiers  are 
used,  the  exact  number  depending  on  the  desired  spread  of  base. 
Each  tier  is  placed  at  right  angles  to  the  one  below  it  and  the 
load  is  carried  to  the  soil  through  beam  action.  The  individual 
beams  of  each  tier  should  be  held  in  place  by  cast-iron  or  gas- 
pipe  separators,  preferably  the  former.  These  separators 
should  be  placed  near  each  end  of  the  beams  and  at  intermediate 
positions  not  over  5  feet  apart.  The  beams  should  be  spaced  so 
as  to  give  a  clearance  of  not  less  than  3  inches,  in  order  that  the 


ART.  153  DESIGN  OF  I-BEAM   GRILLAGES  459 

concrete  may  readily  be  filled  in  between  the  beams;  and  not 
more  than  one  and  one-half  times  the  width  of  the  flange,  in 
order  to  reduce  the  stresses  in  the  concrete  rilling.  The  latter 
requirement  cannot  always  be  met. 

Concrete  should  be  filled  in  between  the  beams  and  also 
placed  around  the  sides,  top,  and  bottom  of  the  grillage.  The 
thickness  of  the  bottom  layer  should  not  be  less  than  1 2  inches, 
and  the  top  and  sides  should  have  a  protective  coating  of  at 
least  4  inches  net  thickness.  If  portland  cement  concrete  is 
used  the  mixture  should  not  be  leaner  than  1-3-6.  A  layer  of 
cement  grout  of  from  \  to  i  inch  in  thickness  should  be  placed 
between  the  tiers  of  beams. 

ART.  153.    DESIGN  OF  I-BEAM  GRILLAGES 

In  designing  a  grillage  the  area  of  the  column  base  and  the 
column  load  will  be  known  in  advance.  The  grillage  will  be 
designed  for  the  same  load  that  is  used  for  the  base  of  the  col- 
umn, namely,  the  total  dead  load  plus  a  certain  percentage  of  the 
live  load,  the  exact  percentage  depending  on  the  kind  of  building 
and  the  number  of  stories.  The  weight  of  the  grillage  itself 
may  usually  be  neglected. 

As  stated  in  Art.  149,  a  moderate  amount  of  settlement  is 
always  to  be  expected,  but  care  should  be  exercised  to  make  this 
settlement  uniform.  To  accomplish  this,  for  any  particular 
case,  the  unit-pressure  on  the  soil  should  be  the  same  for  all 
footings.  This  is  sometimes  difficult  to  obtain  on  account  of  the 
very  considerable  difference  in  the  proportion  of  live  to  dead  load 
for  different  columns. 

Engineers  agree  that  it  is  essentially  the  dead  load  that  causes 
the  settlement:  First,  because  it  always  acts  with  its  maximum 
intensity;  and  second,  because  it  is  the  first  loading  that  comes 
on  the  foundation.  Unequal  settlements  during  erection  due  to 
dead  load,  are  also  troublesome  in  the  case  of  steel  buildings  on 
account  of  the  difficulties  involved  in  fitting  together  the  various 
members  of  the  superstructure.  For  the  above  reasons  most 
engineers  design  footings  for  equal  unit-pressures  under  dead 
load,  or  under  dead  plus  partial  live  loads. 


460  SPREAD   FOUNDATIONS  CHAP.  XV 

In  Table  1530  the  results  are  tabulated  for  the  design  of 
footings  for  an  actual  structure  by  using  four  formulas  that  are 
employed  in  current  practice.  Let  D  denote  the  dead  load  on 
the  base  of  any  column  footing;  T,  total  load  on  same  footing; 
H,  the  dead  load  plus  one-half  the  probable  live  load  on  the 
same  footing;  D'  ',  the  dead  load  on  the  base  of  the  critical  col- 
umn footing;  T',  the  total  load  on  the  base  of  the  critical  col- 
umn footing;  H',  the  dead  load  plus  one-half  the  probable  live 
load  on  the  base  of  the  critical  column  footing;  B,  the  safe  unit 
bearing  value  of  the  soil;  and  A,  the  area  of  bearing  of  base  of 
column  footing  for  that  column  in  which  D  denotes  the  dead 
load.  The  formulas  are  as  follows: 

McCullough,  A=DT'/(BD') 

Schneider,  A=DT'/(BD') 

Moran,  A=HT'/(BHf) 

Live  +  Dead  A 


The  McCullough  formula  gives  uniform  unit-pressure  under 
dead  load,  and  of  a  value  which  makes  the  pressure  under  that 
footing  having  the  minimum  ratio  of  live  to  dead  load  equal 
to  the  safe  bearing  value  of  the  soil.  Thus  the  critical  column 
footing  in  this  formula  is  the  one  having  the  minimum  ratio  of 
live  to  dead  load. 

The  Schneider  formula  gives  uniform  unit-pressure  under 
dead  load,  and  of  a  value  which  makes  the  pressure  under  that 
footing  having  the  maximum  ratio  of  live  to  dead  load  equal  to 
the  safe  bearing  value  of  the  soil.  Thus  the  critical  column 
footing  in  this  formula  is  the  one  having  the  maximum  ratio  of 
live  to  dead  load. 

The  Moran  formula  differs  from  Schneider's  only  in  that 
equal  unit-pressures  occur  under  dead  plus  one-half  probable 
live  load.1  The  other  formula  is  self  evident. 

1  DANIEL  E.  MORAN  explains  the  meaning  of  probable  live  load  as  follows: 
"  The  maximum  probable  load  is  the  load  which  in  the  opinion  of  the  designer  will 
actually  come  upon  the  footings,  and  is  to  be  determined  by  a  study  of  the  con- 
ditions which  will  obtain  when  the  building  is  occupied.  For  instance,  in  a  school- 
house  the  number  of  children  in  each  class  room  and  the  weight  of  desks,  chairs, 
etc.,  may  be  determined  with  considerable  accuracy  and  these  loads  will  make 


ART.  153 


DESIGN   OF   I-BEAM  GRILLAGES 


461 


a 


J>.  cooO    to  CN  oO 


oj  T3  H«  oj  -^ 
0  0  o3  4-  0  c3 
~-  1  ,—  i  O  '  —  i  O 


uumjoQ 


II  II 


•—  ;  3  ^rt  " 

5    rt 

Q    <u    >    0)    <U    > 

H  Q  s  n  Q  3 


O  O 

00  M 

CO 

O  \   00 


-s 


X)    ON  Ovc 

X)    CN   10  O    co  t- 

10  ^t"  M    IO  t~«  M 

11  II      II      II      II  II 


O   Q    ci  -i-  Q 


Q 


W    O  PO  10  ^  ^ 

c-o  t^  10  o  o  o 


ctf  ^  H|IN  rt  ^ 
O  O  «3  -I-  O  cj 
—  '—  i  O  '  —  0 


SN 


uuinjoQ 


10         CN 

to 


a 


uizinjoQ 


8  ^  13 

53  2  §• 
^ 


ill 


lit 


<U     *J 
.>     0) 


ij    M 

3  §3 

o  'd    c 

ir 

BSl 


^  -3   a 

"f  S-l 

5-9  & 


H! 


a,  s 
J=i   bb 


- 


- 


«        3 


462 


SPREAD   FOUNDATIONS 


CHAP.  XV 


The  weak  element  of  the  McCullough  formula  is  that 
although  it  gives  under  dead  load  the  same  unit-pressure  for 
all  foundations,  yet  for  column  No.  24  it  gives  a  pressure  of 
8580  pounds  per  square  foot  for  dead  plus  one-half  live  load  and 
10  740  pounds  per  square  foot  for  dead  plus  live  load,  both  of 
which  are  dangerously  high  when  compared  to  a  safe  value  of 

7000  pounds  per  square 
foot.  The  Schneider  for- 
mula, which  also  gives 
under  dead  load  the  same 
unit-pressure  for  all  found- 
ations, and  a  maximum 
dead  plus  live-load  pres- 
sure of  7000  pounds  per 
square  foot,  is  very  con- 
servative. Neither  of 
these  two  formulas  gives 
any  consideration  to  the 
live  load  in  causing  settle- 


1 

1      *    * 

1 

-— 

... 

....       i     ^ 

r> 

~4 

s      1 

.  j 

!  ! 

i  • 

i     J 

i» 

I  \ 

~<i5> 

!  ! 

! 

i  i 

i  : 

^s> 

1   ; 

i 

!    ! 

1  i 

1    • 

i 

* 

j 

^| 

o 

b 

1 

i 

.....  -3'  6"-  .....  >k—  J'O"—  —> 


—  3'6 


ment.  MORAN'S  formula 
seems  better  in  this  respect 
since  it  recognizes  the  in- 

e 

fluence  of  the  probable  live 
load  and  gives  to  it  one- 
half  the  weight  that  is  given  the  dead  load.     In  it  the  unit- 


FIG.   1530.  —  Steel   I-Beam  Grillage  for  a 

Single  Column. 


pressure  for  dead  plus  one-half  probable  live  load  is  so 
chosen  that  the  maximum  pressure  under  dead  plus  live  load 
equals  the  safe  bearing  power  of  the  foundation  bed.  The  dead- 
plus-live-load  formula  gives  entirely  too  much  weight  to  live 
load,  as  is  seen  from  the  large  variation  in  the  dead-load  stresses. 
For  a  further  discussion  on  this  subject  see  Engineering  News, 
vol.  69,  page  463,  March  6,  1913,  and  page  687,  April  3,  1913. 

the  maximum  probable  live  load.  As  a  further  illustration,  in  many  school- 
houses  there  is  an  assembly  room  which  is  only  used  when  the  class  rooms  are 
vacant  and  consequently  if  class-room  loads  are  used  assembly-room  loads  should 
be  omitted  or  vice  versa;  the  greater  one  of  these  loadings  to  be  used  for  the  prob- 
able load."  A  full  explanation  of  his  method  may  also  be  found  in  the  revised 
edition  of  KEDDER'S  Architects'  and  Builders'  Pocket  Book. 


ART.  153  DESIGN  OF  I-BEAM  GRILLAGES  .         463 

In  designing  steel  grillage  foundations  the  following  assump- 
tions are  made :  First,  the  pressure  from  the  footing  is  uniformly 
distributed  over  the  bed;  second,  the  pressure  of  one  tier  of 
beams  on  another  is  uniformly  distributed  over  the  latter;  third, 
each  tier  acts  independently  of  all  other  tiers;  and  fourth,  the 
concrete  filling  and  covering  carries  no  stress,  acting  merely  as 
a  protection  against  corrosion. 

For  the  single-column  grillage  the  square  base  is  the  most 
economical  shape.  Where  the  possible  width  is  restricted, 
as  in  the  case  of  wall-column  footings,  the  grillage  should  be 
made  as  nearly  square  as.  possible.  Economy  ^  also  results  in 
using  a  minimum  number  of  tiers? 

EXAMPLE  or  DESIGN  or  SINGLE-COLUMN  FOOTING. — Load=6oo  ooo  Ibs. 
Allowable  pressure  on  foundation  bed  =  6000  Ibs.  per  sq.  ft.  Size  of 
column  base  =  3  X4  ft.  Required  area  of  base  =  600  000/6000=  100  sq.  ft. 
A  base  10  ft.  square  is  adopted.  Assume  two  tiers  of  beams.  For  the 
top  tier,  the  maximum  bending  moment  If  =  (600  000/4)  (5  —  2)12.= 
5  400  ooo  Ib.-in.  Using  16  ooo  Ibs.  per  sq.  in.  as  the  safe  unit-stress  in 
the  outer  fiber,  the  total  section  modulus  required  =  I/e  =  5  400  000/16  ooo 
=  337  in3.  Trying  various  combinations  of  beams,  the  following  results 
are  obtained: 

Number      7/ere-      c.        ,,  7/efur-       Width         Clear- 

No.        , ,  .     ,      Size  of  beam         .       , 

of  beams     quired  nished       of  flange        ance 

1  3  112.3        2o"-6s  Ib.          117.0       6. 25  in.       8. 6  in. 

2  4  84.2        i8"-55  Ib.  88.4        6. oo  in.       4.0  in. 

3  5  67-4       I5//~55  Ib.  68.1        5 .75  in.       1.8  in. 

The  choice  lies  between  Nos.  i  and  2,  since  No.  3  does  not  give 
sufficient  clearance.  The  weight  favors  No.  i ,  being  250  pounds 
lighter,  while  No.  2  gives  a  more  satisfactory  clearance  and  has 
less  depth,  thus  saving  on  concrete  filling  and  also  on  excavation 
work. 

Cost  of  250  Ibs.  of  steel  at  2\  cents $6.25 

Cost  of  a  2-in.  thickness  of  concrete $i  .65 


Amount  saved  by  using  design  No.  i $4 . 60 

For  the  lower  tier:  Max.  M  =  (600  000/4)  (5— 1.5)12  =  6  300  ooo  Ib.- 
in.  Total  required  I/e  =  6  300  000/16  000=394  in3.  The  following 
results  are  obtained  by  trying  various  combinations  of  beams: 


464 


SPREAD   FOUNDATIONS 


CHAP.  XV 


No. 

i 

2 

3 
4 


Number 
of  beams 

10 
12 

14 
16 


lie  re- 
quired 

39-4 
32.9 
28.2 
24.6 


Size  of  beam 

i2"-4o    Ib. 
i2"-3ii  Ib. 


I/e  fur- 
nished 

41  .o 
36.0 
36.0 
24.4 


Width 
of  flange 

5.21  in. 
5 .  oo  in. 
5 .  oc  in. 
4.66  in. 


Clear- 
ance 

7 . 5  in. 
5  •  5  in. 
3. 8  in. 
3.0  in. 


io"-25    Ib. 

The  choice  lies  between  Nos.  2  and  4;  the  latter  has  220  pounds  more 
steel  but  the  clearance  is  better  and  a  2-inch  depth  of  concrete  is  saved 

Cost  of  220  Ibs.  of  steel  at  i\  cents  ....................   $5  .  50 

Cost  of  a  2-inch  thickness  of  concrete  ..................   $4  .  50 


Amount  saved  by  using  design  No.  2 . . $i . oo 

ART.  154.     DESIGN  OF  DOUBLE-COLUMN  FOOTINGS 

Where  the  two-column  loads  are  equal  the  base  of  the  footing 
should  be  rectangular  in  shape  and  symmetrical  about  a  line 


~ 

* 

I 

- 

— 

._ 

rj 

--, 

-^ 

-/ 

' 

/ 

& 

... 

... 

... 

... 

1 

1 

GII^J^ 

<- 

-•- 

-" 

: 

.5 

'- 

-> 

<—             --5.5;--                     --> 

.  _9/  K'  _ 

<- 

- 

j. 

-; 

J, 
4< 

-- 

-* 
if* 

4.  75'-  —5 
v 

7T~A" 

I 
I 


1.1 


FIG.   1540. — Double-Column  Footing  of  Steel  I- Beams. 

midway  between  the  columns.  The  total  area  of  the  base  hav- 
ing been  determined  and  the  distance  between  columns  .fixed, 
the  proportion  of  length  to  breath  for  the  base  of  footing  should 
be  such  that  the  moment  in  the  lower  tier  of  beams  under  the 
column  centers  equals  that  at  a  point  midway  between  the  col- 


ART.  154  DESIGN  OF  DOUBLE-COLUMN   FOOTINGS  465 

umns.     This  makes   the   three  maximum  moments   approxi- 
mately equal,  and  gives  the  greatest  economy  of  material. 

EXAMPLE  or  DESIGN  or  DOUBLE-COLUMN  FOOTING,  EQUAL  LOADS. — 
Column  loads  =500  ooo  Ibs.  Column  spacing=i2  ft.  Allowable 
pressure  on  ground  =  4000  Ibs.  per  sq.  ft.  Size  of  column  bases  =  3^X3 
ft.  Allowable  unit-stress  in  beams  =  16  ooo  Ibs.  per  sq.  in.  To  get  the 
value  x  that  will  make  the  three  moments  equal,  500000(6  —  ^/2  = 
500000^/2(6+0;)  — 500  ooo(y76-),  whence  #  =  4.77  ft.  Required  bearing 
area  of  base  =  i  ooo  000/4000=  250  sq.  ft.  Using  a  value  of  #  of  4.75  ft., 
6  =  25o/(i2+2X4.7S)  =  n.63  ft.;  say  11.75.  Let  two  tiers  of  beams 
be  assumed.  Computing  for  top  tier:  Max.  M=  500  000(11.75  — 3)12/8 
=  6  560  ooo  Ib. -in.  Total  required  I/e  =  6  560  000/16  000  =  410  in3. 
After  trying  various  combinations  of  beams,  the  results  are: 

Number      //ere-  .  7/efur-       Width          Clear- 

No.        r,  .     ,     Size  of  beam        .  ,    ,         .„ 

of  beams     quired  mshed       of  flange         ance 

1  3             136.7  24"-8o  Ib.  173-9  7-0    in.  10.5  in. 

2  4             102.5  2o"-65  Ib.  117.0  6.25m.  5-3  in. 

3  5              82.0  i8"-55  Ib.  88.4  6.0    in.  3.0  in. 
No.  2  will  be  adopted. 

For  lower  tier  the  three  positions  of  maximum  bending  moment  are 
at  the  center  and  4.45  ft.  from  each  end.  M  at  center  =  500  ooo 
(6  —  5.375)12  =  3  750000  Ib.-in.  M  at  4.45  ft.  from  the  end  = 

I  500  ooo     4-452       500  ooo     1.4 


Total  required  I/e  =  $  750  000/16000=234.      Upon  trying   various 
combinations  of  beams,  the  results  are  found  to  be: 


No. 

Number 
of  beams 

I/e  re- 
quired 

Size  of  beam 

I/e  fur- 
nished 

Width 
of  flange 

Clear- 
ance 

i 

12 

iQ-5 

9"-2i  Ib. 

18.9 

4-33  in. 

8  .  i  in. 

2 

14 

16.7 

9"-2I  Ib. 

18.9 

4-33  in. 

6.1  in. 

3 

16 

14.6 

8"-i8  Ib. 

14.2 

4.0    in. 

5  .  i  in. 

4 

18 

13.0 

8"-i8  Ib. 

14.2 

4.0    in. 

4.1  in. 

No.  3  will  be  adopted. 

When  the  column  loads  are  not  equal  the  center  of  gravity  of 

the  base  of  the  grillage  is  usually  made  to  coincide  with  the  line 

of  action  of  the  resultant  of  the  two  column  loads  by  making  the 

base  a  trapezoid;  or,  if  the  loads  are  nearly  equal,  it  may  be  done 

30 


466 


SPREAD   FOUNDATIONS 


CHAP.  XV 


by  using  a  rectangular  shape  and  making  the  cantilever  end 
at  the  heavy  load  longer  than  the  other  cantilever  end. 
The  trapezoidal  shape  may  be  obtained  either  by  using  a  larger 
number  of  beams  at  the  heavy  load  end,  or  by  using  the  same 
number  of  beams  and  spacing  them  more  closely  at  one  end  than 
at  the  other.  A  combination  of  the  two  methods  is  sometimes 
used. 


K . 

!< ~ 


*K — -5.55- — ; 

7.125'-- x--2y 

•Q'r87^--~_-----^ 


-**— 3.25'--  -> 


FIG.   1546. — Steel  I-beam  Grillage  for  Two  Columns  Supporting  Unequal  Loads. 
The  load  on  Column  No.  i  is  500  ooo  Ibs;  that  on  No.  2  is  400  ooo  Ibs. 

If  the  proportions  of  the  base  are  so  fixed  that  the  bending 
moment  under  the  center  of  each  column  equals  that  at  the 
center  of  gravity  of  the  base,  the  three  maximum  moments  in 
the  lower  tier  of  grillage  will  be  closely  equal;  this  condition 
gives  approximately  the  minimum  amount  of  material.  The 
most  satisfactory  method  of  determining  the  dimensions  to 
secure  this  result  is  by  trial. 


ART.  154  DESIGN  OF  DOUBLE-COLUMN  FOOTINGS  467 

EXAMPLE  or  DESIGN  OF  DOUBLE-COLUMN  FOOTING,  UNEQUAL  LOADS. — 
Column  loads  and  spacing  as  shown  in  Fig.  154  b.  Allowable  pressure  on 
foundation  bed =4000  Ibs.  per  sq.  ft.  Size  of  column  bases  as  shown  in 
Fig.  I54&.  Allowable  unit-stress  in  beams=  16000  Ibs.  per  sq.  in.  Re- 
quired bearing  area  of  base  =900  000/4000  =2  25  sq.  ft.  Distance  from 
Column  No.  i  to  resultant  of  both  column  loads  =  400  000X10/900  000  = 
4-45  ft. 

After  a  few  trials  it  was  found  that  the  moments  under  the 
centers  of  the  two  columns  and  under  the  center  of  gravity  of 
base — line  of  action  of  resultant  of  two  column  loads — were 
approximately  eqaal  when  ^  =  3.625  ft.,  and  6  =  4.625  ft. 

To  get  b  and  c:  (6+^)18.25/2  =  225  and  (18.25/3)  (b+2c)/(b+c)  = 
3.625+4.45  =  8.075.  Solving  these  two  equations  simultaneously  we 
find  that  approximately  b=  16.6  ft.  and  c  =  8.o  ft. 

Using  two  tiers  of  beams,  the  computations  for  the  upper  tier 
under  Column  No.  i  give:  . 

Max.  M  =(500000/8)  (14.88  —  3)12  =  8  910000  Ib.-in.  Total  required 
//e  =  557  in3-  After  trying  various  combinations  of  beams,  the  results 
are  as  follows,  and  No.  i  is  adopted: 

Number       I /ere-     0.  7/0  fur-       Width         Clear- 

No.        .,  .     ,      Size  of  beam        '        ,         ,- 

of  beams     quired  nished       of  flange        ance 

1  3  186.0        24' '-90  Ib.          186.5        7. 13  in.       7. 3  in. 

2  4  139.5        24"-8olb.          173-9        7-o    in.       2. 7  in. 

For  the  upper  tier  under  Column  No.  2 : 

Max.  M  =  (400 000/8)  (10.18  —  2.75)12  =  4458000  Ib.-in.  Total  re- 
quired 7/^=279  in3.  Trying  various  combinations  of  beams  gives  the 
following  results,  No.  i  being  adopted: 

Number      7/ere-  7/efur-       Width         Clear- 

No.        ,,  .     ,      Size  of  beams        .  ,     ,         ._ 

of  beams     quired  nished       of  flange        ance 

1  3  93.0          i8"-6o  Ib.  93.5        6. 10  in.       7. 3  in. 

2  4  69.7          i8"-55  Ib.  88.4        6. oo  in.       3.0  in. 

In  designing  the  lower  tier  and  running  all  beams  full  length, 
let  #  =  the  distance  from  the  left  end  of  grillage  to  section  in 
question,  the  expression  for  bending  moments  under  Column 
No.  i,  between  the  two  columns,  and  under  Column  No.  2, 
are  respectively  as  follows: 


468  SPREAD   FOUNDATIONS  CHAP.  XV 

4000     X2  500000      0-2. 125)2 

If  (col.   No.i)  = (49.8  —  0.471^)—— 

23  32 

2 

M  (between  cols.)  = •  (49.8  — 0.471^)  — 500000^— 3.625) 

2        3 

M(col.  No.  2)= •—•(49.8  — 0.47 1#)  — 

2       3 

400000     (S-I2.25)2 

500  000(^-3.6-25) 

To  get  the  values  of  x  for  the  maximum  value  of  M  in  each  of 
the  above  equations  equate  dM/dx  equal  to  zero,  which  gives 
3.42,  8.58  and  13.91  ft.,  respectively.  Substituting  these  three 
values  of  x  in  the  preceding  equations,  the  corresponding  values 
of  M  are,  236000  lb.-in.,  231000  lb.-in.,  and  232000  Ib.-in. 
The  maximum  maximorum  is  therefore  236  ooo  lb.-in.  Total 
required  I/e=  177  in.3  Trying  various  combinations  of  beams, 
there  is  obtained: 


No. 

Num- 
ber of 
beams 

I/e  re- 
quired 

Size    of 
beam 

I/e 
fur- 
nished 

Width 
of 
flange 

Clearance 

i 

10 

17.7 

9"-2I      Ib. 

18.9 

4-  33  in. 

5.9  to  17.3  in. 

2 

12 

14.7 

8//       i  11 
—  2Oj  ID. 

15.0 

4  .  08  in. 

4.3  to  13.6  in. 

3 

14 

12.7 

8"-i8    Ib. 

14.2 

4  .  oo  in. 

3.,i  to  10.9  in. 

U 

16 

II.  0 

7"-i7^1b. 

II.  2 

3.  66  in. 

2.  5  to    9.  3  in. 

No.  4  will  be  adopted. 

Reinforcing  bars  should  be  placed  in  the  concrete  near  the 
upper  surface  for  the  wider  half  of  the  footing. 


ART.  155.     DISTRIBUTION  or  PRESSURE  ON  BASE 

There  is  some  question  regarding  the  error  involved  in  the 
assumption  that  the  pressure  from  the  footing  is  uniformly 
distributed  on  the  ground.  Taking  the  case  of  the  single- 
column  square  footing  it  is  evident  that  the  base  of  the  footing 
will  assume  a  saucer-like  shape,  and  as  a  consequence  the  pres- 
sure will  be  a  maximum  at  the  center  and  a  minimum  at  the 
outside.  The  law  governing  the  variation  of  pressure_will 


ART.  156  STEEL  GRILLAGE  FOUNDATIONS  469 

depend  on  the  relative  deflections  of  different  points  on  the 
base  of  the  footing,  as  well  as  on  the  modulus  of  compressibility 
of  the  soil  and  the  thickness  of  the  compressible  stratum. 
Where  the  modulus  is  low  and  the  thickness  considerable,  the 
slight  difference  in  total  deformation  at  different  points  will 
cause  but  a  slight  difference  in  pressure.  Where  the  soil  is 
compressible  but  inelastic,  or  soft  and  subject  to  lateral  flow, 
a  fairly  uniform  distribution  of  pressure  quickly  obtains. 

Where  the  material  has  a  high  modulus  of  compressibility, 
as  in  shale  or  rock,  the  footing  should  be  designed  for  stiff- 
ness as  well  as  for  strength  or  else  the  surface  of  the  material 
should  be  shaped  to  fit  the  curve  taken  by  the  base  of  the  foot- 
ing when  fully  loaded,  otherwise  the  pressure  will  be  very  un- 
evenly distributed.  For  example,  by  using  a  stress-strain 
diagram  of  the  values  obtained  in  the  foundation  tests  of  the 
St.  Paul  Building,  New  York  City  (see  Engineering  Record, 
vol.  33,  page  388,  May  2,  1896),  a  theoretical  solution  shows 
that  for  the  typical  steel-grillage  footing  the  pressure  varies 
from  a  maximum  at  the  center  to  approximately  zero  at  the 
outside.  The  material  on  which  the  above  foundation  tests 
were  made  consisted  of  very  compact  sand,  while  the  whole 
area  of  the  lot  was  covered  with  a  layer  of  concrete  and  steel 
beams  buried  in  concrete,  the  tests  being  made  through  a  hole. 

ART.  156.     STEEL  GRILLAGE  FOUNDATIONS 

Most  of  the  grillages  used  in  the  foundations  for  the  Phelan 
Building,  San  Francisco,  were  15  feet  square,  and  made  with 
two  cross  tiers  of  I-beams  from  1 8  to  24  inches  in  depth,  or  with 
an  upper  tier  of  built-up  girders  and  a  lower  tier  of  I-beams, 
as  shown  in  Fig.  1560.  The  complete  grillage  plan  is  shown 
in  Fig.  1566. 

"All  footings  are  made  with  a  bed  of  concrete  12  inches 
thick  and  12  inches  wider  and  longer  than  the  dimensions  of 
the  first  tier  of  grillage  beams.  In  the  upper  part  of  the  con- 
crete there  are  two  full-length  rectangular  grooves  transverse 

1  Engineering  Record,  vol.  57,  page  366,  March  28,  1908. 


470 


SPREAD   FOUNDATIONS 


CHAP.  XV 


to  the  lower  tier  of  grillage  beams.     In  each  groove  a 
TV-inch  angle  was  carefully  leveled  with  the  upper  edge  of  its 
vertical  flange  truly  horizontal  and  f  inch  above  the  surface 


Boiler RoomFT 


FIG.   1560. — Footings  with  Plate  Girders  and  I-beams  in  Double  Tiers. 

of  the  concrete.  These  serve  as  leveling  bars  to  receive  the 
lower  flanges  of  the  grillage  beams  and  insure  their  exact  height. 
The  spaces  between  the  beams  and  the  concrete  footings  were 
grouted,  the  second  tier  of  beams  was 
shimmed  f  inch  above  the  top  flanges 
of  the  lower  tier  and  grouted,  the  cast- 
iron  pedestals  were  set  f 
inch  above  the  top  flan- 
ges of  the  distributing 
beams  and  grouted,  and 
a  solid  mass  of 
concrete  was 


FIG.   1566. — Grillage  Plan  of  Phelan   Building,  San  Francisco,  Cal. 

filled  in  6  inches  around  the  outer  edges  of  the  beams  and 
pedestals  and  up  to  the  cellar  floor,  completely  enclosing  and 
protecting  all  the  substructure  steel  work." 


ART.  156 


STEEL  GRILLAGE  FOUNDATIONS 


471 


Fig.  156  c  illustrates  a  very  heavy  grillage  foundation  for 
four  columns  of  the  Curtis  Building,  Philadelphia.  It  was 
necessary  to  use  a  single  grillage  for  the  four  columns  because 
of  the  short  distances  between  the  latter.  The  distributing 
girders  for  Columns  Nos.  254  and  255  have  48XiHnch  webs 


r 


-a4-  is 


\S  NX  v/  \ 


ing  ffoda . 
FIG.   i  s  6c.— Special   Footing   for   Four    Columns,    Curtis    Building,  Philadelphia. 

reinforced  by  5X3Xf-mcn  vertical  stiflener  angles  and  two 
i3Xi-inch  vertical  side  plates,  and  the  top  flanges  of  the 
girders  are  connected  by  transverse  tie  plates.  The  column 
loads  are  transmitted  to  the  triple  distributing  girders  by 
bolsters  made  of  solid  slabs  of  plain  square  steel  billets 


472 


SPREAD    FOUNDATIONS 


CHAP.  XV 


which  are  bolted  to  the  upper  flanges  of  the  girders.  The 
concrete  footing  is  reinforced  with  rods  for  part  of  the 
base,  due  to  the  fact  that  the  I-beams  are  there  a  con- 
siderable distance  apart,  thus  developing  beam  action  in  the 
concrete. 


Barclay 


9  tO 


Place 


FIG.   156^.  —  Plan  of  Piers   and  Grillages  for  the  Woolworth    Building. 


The  Woolworth  Building,  New  York  City,  is  founded  on 
solid  rock  115  feet  below  the  curb  level.  The  loads  are  car- 
ried from  the  columns  to  bedrock  through  grillage  footings 
resting  on  reinforced-concrete  piers.  Fig.  156^  shows  the 
general  lay  out  for  the  foundation,  while  1560  shows  some  of 
the  details. 


ART.  156 


STEEL   GRILLAGE   FOUNDATIONS 


473 


474 


SPREAD   FOUNDATIONS 


CHAP.  XV 


ART.  157.  DESIGN  OF  REINFORCED-CONCRETE 
SPREAD  FOUNDATIONS 

Instead  of  serving  merely  as  a  protection  for  the  steel,  con- 
crete may  be  made  to  take  a  part  of  the  load  by  using  a  rein- 
forced-concrete  footing  in  place  of  the  I-beam  grillage,  thus 
lessening  the  cost  of  the  foundation.  Another  advantage  pos- 
sessed by  a  reinforced-concrete  foundation  is  that  it  can  be 
cast  in  any  shape  or  form  desired.  It  may  be  in  the  form  of  a 
flat  slab  or  of  the  slab-and-beam  type  (Fig.  1590).  The  former 

uses  more  concrete,  while  in 
the  latter  the  form  work  is 
more  expensive.  For  some 
interesting  modifications  of 
the  elementary  type  the 
reader  is  referred  to  Art.  159. 
DESIGN  OF  A  REINFORCED- 
CONCRETE  WALL  FOOTING.  — 
Assuming  the  load  to  be 
64  ooo  pounds  per  linear  foot 
of  wall  and  the  allowable 
bearing  on  the  soil  4000 
pounds  per  square  foot,  the 
width  of  footing  will  be  64000/4000=16  feet.  The  thick- 
ness of  the  wall  is  2  feet  (Fig.  1570).  The  footing  will  be 
designed  at  three  sections,  at  a,  5^  feet  from  the  center  of  the 
wall,  at  b,  3  feet  from  the  center,  and  at  c,  i  foot  from  the 
center.  Taking  a  i-foot  length  of  footing  the  vertical  shears 
and  bending  moments  will  be  as  follows: 

Va  =4000X21  =  10  ooo  lb.        Ma  =  ""  =  150  ooo  Ib.  -in. 


FIG. 


-Reinforced-concrete  Wall 
Footing. 


** 
=4000X5  =20  ooo  lb.        M 


4°o°X 


. 
000000  lb.  -in. 


Vc  =4000X7   =  28  ooo  lb.        Mc  = 


4°°°X 


=i  176  ooo  Ib.-in. 


ART.  157     REINFORCED-CONCRETE  SPREAD  FOUNDATIONS  475 

A  1-2-4  concrete  will  be  used,  with  an  allowable  compressive 
unit-stress1  in  the  concrete  of  fc  =  600  Ibs.  per  sq.  in.  and  an 
allowable  tensile  unit-stress  in  the  steel  of  fs=i6  ooo  Ibs.  per 
sq.  in.  The  ratio  of  the  modulus  of  elasticity  of  steel  to  that  of 
concrete  will  be  assumed  as  ^=15.  The  depth  to  center  of 
steel  rods  necessary  to  give  a  compressive  stress  in  the  concrete 
of  600  Ibs.  per  sq.  in.  is  given  by  the  formula,  d  =  ^lM/(Rb),  in 
which  R  =  fckj/2.  In  the  latter  formula2  k  =  ^2pn-\-(pn)2—pn 

and  j  =  i  —  &/3  5  p  =  |/y ( -~  +  i ) .     The  work  involved  in  get- 
Jc  \njc         / 

ting  the  value  of  R  will  be  greatly  reduced  by  using  the  dia- 
grams found  in  TURNEAURE  and  MAURER'S  Reinforced-Con- 
crete  Construction.  For  the  problem  at  hand  the  value  of 
R  is  95.  Solving  for d,  da  =  V 150 0007(95X1 2)  =  ii.5in.;J& 
=  V  6000007(95X12)  =23.0 in.; and  dc=  V  i  176000/95X12) 
=  32. i  in.  (32^  in.  being  adopted).  As  it  is  inadvisable  to  use 
a  depth  at  any  section  less  than  about  6  inches  the  form  shown 
in  Fig.  157  a  will  be  adopted.  The  steel  in  the  bottom  will  be 
given  a  2 -in.  insulation. 

The  area  of  steel  required  at  each  section  is  given  by  the 
formula,  A=M/(fsjd).  Using  the  values  of  d  obtained  above, 
so  that  the  footing  be  equally  strong  in  tension  and  compression: 

Aa  =    150  ooo/(i6  000X0.88X11. 5)  =  o.Q2  square  inch, 
Ab  =    600  ooo/(i6  000X0.88X23     )  =  1.8 5  square  inches, 
Ac  =i  176  ooo/(i6  000X0.88X32.1)  =  2. 60  square  inches. 

Using  a  rod  spacing  of  3  inches  center  to  center  there  will  be 
4  rods  in  one  foot  of  length  of  the  footing.  The  required  area 
of  each  rod  will  be  2.60/4  =  0.650  sq.  in.  A  le-inch  square 

1  In  a  wedge-shaped  beam  the  greater  principal  stress  at  the  outer  fibers  acts 
parallel  to  the  upper  surface  of  the  beam  and  with  an  intensity  equal  to  the  maxi- 
mum normal  stress  on  a  vertical  plane  divided  by  cos2  a,  in  which  a  is  the  angle 
of  inclination  of  the  upper  surface  of  the  beam;  hence,  the  allowable  bending  unit 
stress  should  be  taken  equal  to  the  safe  compressive  stress  in  the  concrete  multi- 
plied by  cos2  a. 

2  Based  upon  the  assumption  that  the  normal  stress  in  the  concrete  on  any 
vertical  section  varies  as  a  straight  line  and  that  the  stress  in  the  steel  equals 
n  times  the  stress  in  the  concrete.     For  formulas  based  on  a  different  assumption 
see  Proc.  Am.  Soc.  C.  E.,  vol.  39,  page  2067,  Nov.  1913. 


47 6  SPREAD   FOUNDATIONS  CHAP.  XV 

twisted  rod,  giving  an  area  of  0.660  sq.  in.  will  be  adopted. 
Three  rods  will  furnish  the  required  area  at  b,  while  two  rods  will 
furnish  that  required  at  a;  hence  certain  of  the  rods  may  be 
bent  up  or  cut  off  as  shown  in  Fig.  1570. 

Using  an  allowable  bond  unit-stress  of  140  Ibs.  per  sq.  in. 
of  rod  surface  the  necessary  length  of  rod  to  develop  full  strength 
is  (16  000X0.66)7(140X3. 25)  =  23. 2  in.  Computing  the  bond 
stress  in  the  rods  by  the  formula  1u=(Sd  —  M  tan  a)/(jd2  20) 
in  which  tan  a  is  the  slope  of  the  upper  surface  of  the  footing 
and  20  the  perimeter  of  the  rods  at  the  section  in  question,  the 
values  are  as  follows: 


w0  =  (10000X15.4  — !5ooooXo.3i2)/(o.88Xi5.42  X6.5o)  =  79lb./sq.  in. 
ub  =  (20000X24.75  — 600000X0.312)7(0.88X24. 752X9.75)  =  59lb./sq.  in. 


uc  =(28000X32.25  —  1  176000X0. 312)7(0. 88X32. 252Xi3)  =  45lb./sq.  in. 

All  of  these  values  are  well  below  the  safe  limit  of  140  Ibs. 
per  sq.  in. 

Assuming  that  the  concrete  takes  no  longitudinal  tension 
the  maximum  intensity  of  diagonal  tension  is  given  by  the 
formula  t=(Sd—M  tan  a  )/(jd2).  A  shorter  method  of  com- 
puting the  maximum  diagonal  tension  is  by  taking  the  bond 
stress  values  and  multiplying  them  by  the  perimeter  of  the  rods. 
Thus, 

ta  =(79X6.50)712  =  43  Ibs.  per  sq.  in. 
fa  =(59X9.75)712  =  48  Ibs.  per  sq.  in. 
lc  =(45X13.0  )/i2  =  49  Ibs.  per  sq.  in. 

Although  conservative  specifications  limit  the  allowable 
diagonal  tension  to  40  pounds  per  square  inch,  the  above  can 
be  safely  carried  by  the  concrete  without  reinforcement,  but 
to  illustrate  the  method  stirrups  will  be  designed  to  carry  all 
of  this  tension.  Placing  the  stirrups  on  a  45-degree  slope  and 
using  f-inch  square  twisted  rods  with  two  prongs  in  a  1 2-inch 
length,  as  shown  in  Fig.  1570,  the  strength  of  one  line  of 
stirrups  in  a  1 2-inch  length  will  be  16  oooX(|-)2X2  =  45  ooo 
pounds.  Denoting  the  horizontal  distance  between  rows  of 

1  Only  approximately,  true  when  p  is  not  constant. 


ART.  158       REINFORCED-CONCRETE    COLUMN   FOOTINGS 


477 


stirrups  by  5  the  formula  is,  s  =  45oo/(i2X/Xcos  45°),  giving 

Sa  =4500/12X43X0.707  =  12  inches. 
Sb  =4500/12X48X0.707  =  11  inches. 
Sc  =4500/12X49X0.707  =  10  inches. 

A  uniform  spacing  of  10  inches  will  be  adopted. 

In  this  type  of  beam  the  maximum  intensity  of  vertical 
shear  occurs  at  the  top  and  equals  fc  tan  a,  where  a  is  the  in- 
clination of  the  upper  surface  of  the  slab.  The  shearing  stress 
is  therefore  600X0.312  =  187  pounds  per  square  inch. 


ART.  158.  DESIGN  OF  REINFORCED-CONCRETE 
COLUMN  FOOTINGS 

The  stresses  in  a  reinforced-concrete  footing  for  a  column  are 
due  more  to  flat-slab  action  than  to  beam  action  and  hence  are 
much  less  determinate  than  in  the 
wall  footing.  However,  the  stresses 
may  be  approximately  analyzed  by 
either  flat- slab  or  beam  formulas. 
The  former  method  is  not  entirely 
satisfactory,  due  partly  to  the  neces- 
sary approximations  of  any  formulas 
based  on  the  theory  of  the  flat 
plate,  and  partly  to  the  tedious  com- 
putations; involved  unless  specially 
prepared  tables  or  diagrams  are  used. 
For  an  example  of  the  design  of  a 
footing  based  on  the  flat-slab  prin- 
ciple see  page  644  of  the  second  edition  of  TAYLOR  and  THOMP- 
SON'S Concrete,  Plain  and  Reinforced. 

Where  beam  formulas  are  used  it  is  generally  assumed  that 
the  section  of  maximum  bending  moment  and  shear  is  at  the 
outer  face  of  the  column.  If  the  footing  has  a  two-way  rein- 
forcement the  stress  cannot  be  uniformly  distributed  over 
this  section.  For  instance,  looking  at  Fig.  158  a,  the  load  from 
the  soil  at  point  c  will  evidently  go  to  the  column  through  dc 


FIG.   1580. — Column  Footing  of 
Reinforced  Concrete. 


SPREAD  FOUNDATIONS  CHAP.  XV 

acting  as  a  cantilever  beam.  On  the  other  hand  a  part  of  the 
load  at  a  will  first  go  to  some  point  as  c  through  ac  acting  as  a 
beam,  and  the  balance  to  some  point  as  b  through  ab  acting  as 
a  beam.  The  part  which  goes  to  c  will  then  go  to  d  through 
cd  acting  as  a  beam,  while  the  part  which  goes  to  b  will  go  to  e 
through  be  acting  as  a  beam.  Thus  it  is  evident  that  the  stress 
along  the  plane  A- A  will  vary  from  a  maximum  at  the  column 
face  to  a  minimum  near  the  sides  of  the  footing. 

From  experiments  made  in  the  testing  laboratory  at  the 
University  of  Illinois,  A.  N.  TALBOT  summarizes  the  proper 
method  of  design  as  follows:  luFor  footings  having  projec- 
tions of  ordinary  dimensions,  the  critical  section  for  the  bending 
moment  for  one  direction  (which  in  two-way  reinforced  con- 
crete footings  is  to  be  resisted  by  one  set  of  bars)  may  be  taken 
to  be  at  a  vertical  section  passing  through  the  face  of  the  pier. 
In  calculating  this  moment,  all  the  upward  load  on  the  rectangle 
lying  between  a  face  of  the  pier  and  the  edge  of  the  footing 
is  considered  to  act  at  a  center  of  pressure  located  at  a  point 
halfway  out  from  the  pier,  and  half  of  the  upward  load  on  the 
two  corner  squares  is  considered  to  act  at  a  center  of  pressure 
located  at  a  point  six- tenths  of  the  width  of  the  projection  from 
the  given  section.  .  .  . 

"With  two-way  reinforcement  evenly  spaced  over  the  foot- 
ing, it  seems  that  the  tensile  stress  is  approximately  the  same 
in  bars  lying  within  a  space  somewhat  greater  than  the  width 
of  the  pier  and  that  there  is  also  considerable  stress  in  the  bars 
which  lie  near  the  edges  of  the  footing.  For  intermediate 
bars  stresses  intermediate  in  amount  will  be  developed.  For 
footings  having  two-way  reinforcement  spaced  uniformly  over 
the  footing,  the  method  proposed,  for  determining  the  maxi- 
mum tensile  stress  in  the  reinforcing  bars,  is  to  use  in  the  cal- 
culation of  resisting  moment  at  a  section  at  the  face  of  the 
pier  the  area  of  all  the  bars  which  lie  within  a  width  of  footing 
equal  to  the  width  of  pier  plus  twice  the  thickness  of  footing, 
plus  half  the  remaining  distance  on  each  side  to  the  edge  of  the 
footing.  This  method  gives  results  in  keeping  with  the  results 

1  Bulletin  No.  67,  Engineering  Experiment  Station,  University  of  Illinois. 


ART.  158       REINFORCED-CONCRETE    COLUMN   FOOTINGS 


479 


of  tests.  When  the  spacing  through  the  middle  of  the  width 
of  the  footing  is  closer,  or  even  when  the  bars  are  concentrated 
in  the  middle  portion,  the  same  method  may  be  applied  without 
serious  error.  Enough  reinforcement  should  be  placed  in  the 
outer  portion  to  prevent  the  concentration  of  tension  cracks  in 
the  concrete  and  to  provide  for  other  distribution  stresses. 

"The  method  proposed  for  calculating  maximum  bond  stress 
in  column  footings  having  two-way  reinforcement  evenly  spaced, 
or  spaced  as  noted  in  the  pre- 
ceding paragraph,  is  to  use  the 
ordinary  bond-stress  formula, 
and  to  consider  the  circumfer- 
ences of  all  the  bars  which  were 
used  in  the  calculation  of  tensile 
stress,  and  to  take  for  the  exter- 
nal shear  that  amount  of  upward 
pressure  or  load  which  was  used 
in  the  calculation  of  the  bending 
moment  at  the  given  section." 
In  the  preceding  discussion  the 
slab  is  assumed  to  have  a  hori- 
zontal upper  surface. 

DESIGN  OF  A  FOUR- WAY  RE- 
INFORCED FOOTING. — A  footing 
with  four-way  reinforcement 
(Fig.  1586)  is  more  susceptible 
of  a  rational  analysis  than  the  two-way  reinforced  foot- 
ing. Tests  by  A.  N.  TALBOT  (see  previous  reference)  show 
that  this  type  gives  a  somewhat  stronger  footing  than  the 
two-way  type. 

Assuming  the  load  to  be  210000  pounds  and  the  allowable 
bearing  on  the  soil  3000  Ibs.  per  sq.  ft.,  the  area  of  the  footing 
will  be  210  000/3000=70  sq.  ft.  A  baseS  feet  6  inches 
square  will  be  used.  The  column  base  will  be  assumed  as  20 
inches  square. 

In  this  design  the  part  A  BCD  in  Fig.  158  b  will  be  assumed 
to  act  as  a  free  cantilever  about  CD,  as  will  also  ABEF,  ABGH 


FIG.   1586. — Reinforcement  for 
Column  Footing. 


480  SPREAD   FOUNDATIONS  CHAP.  XV 

and  ABKL  about  EF,  GH  and  KL  respectively;  in  other 
words,  it  will  be  assumed  that  there  is  no  stress  on  the  planes 
AD  and  BC.  Dividing  the  horizontal  distance  between  AB 
and  DC  into  four  equal  parts  by  the  lines  &i,  62  and  £3,  the  lengths 
of  the  lines  bQ,  bi,  b2,  bs  and  Z>4  are  respectively  8.50,  6.79,  5.08, 
3.37  and  1.67  ft.  Let  AI,  A2)  As  and  A±  represent  respectively 
the  areas  of  the  base  of  the  footing  to  the  right  of  the 
b  lines  of  the  corresponding  subscripts,  then  their  values  will 
be  Ai=  6.54,^2=11.6,^43  =  15.2  and  ^4=17.35,  all  expressed 
in  square  feet. 

The  upward  pressure  from  the  soil  is  2ioooo/(8.5)2=2gio 
Ibs.  per  sq.  ft.  The  shears  on  the  sections  bi,  62,  bs  and  £4  are 
respectively  19  ooo,  33  ooo,  44  200  and  50  500  pounds.  The 
moment  of  the  upward  pressure  to  the  right  of  and  about  b\ 

is    19  oooX  2><8-5+6-79  x  gj54  Xi2  =  ioiooo    Ib.-in.     The 

8.5+6.79  3 

moments  of  the  forces  to  the  right  of  and  about  bz,  bs  and  b± 
are  respectively  376000,  775000,  and  i  267000  Ib.-in.  Using 
an  allowable  unit  stress  for  the  rods  of  16000  Ibs.  per  sq.  in. 
and  for  the  concrete  of  650  cos2  a  =  500  (approximately)  Ibs. 
per  sq.  in.,  in  which  a  is  the  angle  made  by  the  upper  surface 
with  the  horizontal,  the  values  of  d  as  given  in  the  [formula 
d  =  M/(Rb)  are  ^1  =  4.2,  J2  =  9-3,  ^3=16.4  and  ^4  =  29.8  in. 

Using  the  formula  A=M/(fsjd)  to  get  the  required  area  of 
cross-section  of  steel  at  bi,  bz,  b$  and  64,  the  respective  values 
are  1.69,  2.83,  3.30  and  2.97  sq.  in.  Assuming  12  square 
twisted  rods,  the  required  area  of  each  one  is  3.30/12  =  0.275 
sq.  in.  A  iVinch  rod  furnishes  an  area  of  0.316  sq.  in.  The 
rods  will  be  placed  as  shown  in  Fig.  158^,  each  layer  being 
i^  in.  above  the  one  below  it. 

The  ordinates  to  the  curved  line  in  Fig.  1586  represent  the 
required  depths,  but,  as  shown  in  the  same  illustration,  the 
depths  adopted  will  be  greater  than  these. 

The  bond  stresses  as  given  by  the  formula  u=(Sd—M  tan  «)/ 
(jd22o)  are  48,  50,  44  and  36  Ibs.  per  sq.  in.  for  the  sections 
bij  bz,  &3,  and  64  respectively. 

The  maximum  unit  shear  is  fc  tan  01  =  500X0.586  =  293  Ibs. 


ART.  159        CONCRETE  SPREAD  FOUNDATIONS  481 

per  sq.  in.  This  is  a  rather  high  value  but  as  it  occurs  at  the 
point  of  maximum  compression  and  so  does  not  develop  a 
heavy  diagonal  tension,  it  may  be  considered  safe. 

Assuming  that  the  concrete  takes  no  direct  tension  the 
maximum  diagonal  tension  for  each  section,  as  given  by  the 
formula  t=(Sd  —  M  tan  a)/(bjd2),  is  /i=i6,  /2  =  22,  £3  =  29  and 
£4=48  Ibs.  per  sq.  in.  Hence  stirrups  are  required  for 
only  a  short  distance  from  the  face  of  the  column.  The 
method  of  design  of  the  same  is  treated  in  Art.  157  and  will 
not  be  repeated  here. 

The  design  of  the  slab-and-beam  type  of  footing  follows 
closely  the  method  of  design  of  slabs  and  beams  in  building 
construction.  The  slab  serves  as  a  beam,  to  carry  the  load 
from  the  soil  to  the  beam,  the  span  being  taken  as  the  distance 
center  to  center  of  beams;  and  the  latter,  acting  as  cantilevers, 
carry  it  to  the  column.  Where  the  beams  have  constant  cross- 
sections  the  formulas  for  stresses  as  derived  in  any  standard 
treatise  on  reinforced  concrete  are  applicable,  and  where 
tapered,  the  formulas  given  in  Art.  157  may  be  used.1 

Where  one  footing  serves  for  two  columns,  the  method  of 
obtaining  the  shape  of  footing,  as  well  as  the  shears  and  bending 
moments,  is  similar  to  that  for  the  I-beam  grillage  (Art.  154), 
while  the  standard  formulas  are  applicable  in  finding  the  stresses. 
On  page  647  of  the  second  edition  of  TAYLOR  and  THOMPSON'S 
Concrete,  Plain  and  Reinforced,  an  example  of  this  type  of  foot- 
ing is  worked  out. 

ART.  159.     CONCRETE  SPREAD  FOUNDATIONS 

Two  standard  forms  of  the  reinforced-concrete  spread  foot- 
ings used  for  the  column  foundations  of  a  railway  terminal  sta- 
tion at  Atlanta,  Ga.,  are  shown  in  Fig.  i59<x.  The  one  illus- 
trated on  the  left  was  used  for  20  X  24-inch  columns  and  was 
in  the  form  of  a  truncated  pyramidal  slab  reinforced  with 
bars  and  stirrups.  The  one  shown  on  the  right  was  of  the 
beam-and-slab  type.  The  details  are  sufficiently  shown  to 
require  no  explanation. 
31 


482 


SPREAD    FOUNDATIONS 


CHAP.  XV 


The  125-foot  concrete  block  chimney  for  the  St.  Joseph's 
Home,  Chicago,  was  founded  on  a  blue  clay,  the  base  of  the 
foundation  extending  about  5  feet  below  the  surface  of  the 
ground.  The  footing,  shown  in  Figs.  159^  and  c,  consists 
of  a  circular  slab  24  feet  in  diameter  and  10  inches  thick,  on 
which  is  built  a  box  with  a  square  outer  surface  8  feet  3  inches 
on  a  side,  and  with  an  octagonal  inner  surface  about  6  feet  7 


8.l?"Vert!cal  Rods 
Binders  every  10" 


Elevation. 

//&•— 

:j"Rods,7"C.1oC. 


J_ 


Rod 


Section  B-B. 


FIG. 


.  —  Pyramidal  and  Ribbed  Slab  Footings  of  Reinforced  Concrete,  Atlanta 
Terminal  Station,  Southern  Railway. 


inches  between  opposite  faces.  The  box  is  about  4  feet  high. 
luFrom  either  corner  of  this  box  extends  a  series  of  eight 
cantilever  ribs  reaching  approximately  to  the  outer  edge  of  the 
slab  as  shown  in  the  accompanying  view.  These  cantilever 
ribs  are  each  14  inches  wide,  4  feet  deep  at  the  box  and  slope 
off  uniformly  to  a  width  of  8  inches  at  the  top  of  the  slab. 


Engineering  Record,  vol.  65,  page  636,  June  8,  1912. 


ART.  159 


CONCRETE  SPREAD  FOUNDATIONS 


483 


FIG.   1596. — Slab  and  Box  Footing  of  Reinforced  Concrete  for  a  1 25-foot  Chimney 

in  Chicago. 


FIG.   i59c. — View  of  the  same  Footing  as  shown  in  Fig.  1596. 


484 


SPREAD  FOUNDATIONS 


CHAP.  XV 


Their  effective  depth  is  virtually  4  feet  plus  the  effective  depth 
of  the  slab,  as  they  are  built  integral  with  it;  and  their  rein- 
forcement, which  consists  of  i-inch  round  bars  and  J-inch  ver- 
tical stirrups,  extends  up  from  the  lower  surface  of  the  slab,  as 
shown  in  the  accompanying  drawing.  The  base  is  thus  made 
up  of  a  series  of  slabs,  each  supported  by  the  adjacent  canti- 
lever ribs  and  reinforced 
with  f-inch  round  bars, 
spaced  according  to  the 
position  of  the  slab  in  the 
base.  That  portion  of 
the  base  enclosed  at  the 
center  is  reinforced  with 
a  double  system  of  J-inch 
round  bars,  spaced  6| 
inches  on  centers." 

Fig.  159^  illustrates  a 
reinf  orced-concrete  footing 
on  a  pile  foundation,  Fig. 
1590  represents  a  novel 
type  of  foundation  used 
for  a  loft  building  in  New 
York  City.  There  were 
three  lines  of  columns,  two 
lines  of  wall  columns  and 
one  line  through  the  cen- 
ter. The  foundation  pre- 


Rinqs 
Welded  y 


\ 

? 

^  i 

\ 

S   .-fastened 

"'"  '1 

\                              8,  \°'  Shear/ 

r       -_        ^J=^=*^  =  <=-^_--"--  Iggg  —  p™' 

^ 

£|T~i 

i             rrn  i 

!  LL 

J             LLU 

L 

JJ  ; 

FIG.  1590?. — Column  Footing  of  Reinforced 
Concrete  Supported  by  Pre-molded  Concrete 
Piles. 


sented  something  of  a 
problem  because  the  ad- 
joining structure  rested 

on  a  pile  foundation,  which  the  architect  feared  was  in  a  poor 
condition.  On  account  of  the  desire  not  to  be  forced  to  the 
expense  of  underpinning  this  adjoining  building,  a  deep  founda- 
tion was  out  of  the  question.  The  simple  spread  footing  could 
not  be  used  for  the  wall  columns  because  of  lack  of  space.  As 
finally  constructed,  the  foundation  consisted  of  a  solid  frame- 
work of  reinforced-concrete  beams. 


ART.  159 


CONCRETE   SPREAD  FOUNDATIONS 


485 


1  "The  special  feature  of  the  cantilever  construction  is  that 
the  one  cross-beam  and  a  portion  of  each  longitudinal  beam 
form  a  T-section,  the  center  of  gravity  of  which  is  the  same 
as  the  center  of  gravity  of  the  column  loads  plus  the  weight  of 
the  side  wall.  Thus,  looking  at  Fig.  1590,  it  will  be  seen  that 
half  of  the  load  coming  on  the  column  in  the  center  of  the  build- 
ing and  the  whole  load  coming  on  a  wall  column  and  the  wall 


EN6.  NEWS 


FIG.   159^. — Plan   of    Reinforced-Concrete    cantilever    Footings   of    1 2-story   Loft 
Building,  25-29  West  3ist  Street,  New  York  City. 

load  adjacent  to  that  column  is  carried  on  that  portion  of  the 
side  concrete  beam  and  the  cross-beam  there  shown,  and  that 
the  center  of  gravity  of  these  loads  is  the  same  as  the  center 
of  gravity  of  the  T-beam  formed  by  the  side  beam  with  the 
transverse  beam  going  at  right  angles  from  it.  The  variation 
in  the  loads  and,  consequently,  in  the  centers  of  gravity,  re- 
sulted in  different  shapes  and  sizes  of  the  supporting  beams." 

1  Engineering  News,  vol.  68,  page  995,  Nov.  28,  1912. 


486 


SPREAD   FOUNDATIONS 


CHAP.  XV 


Reinforced-concrete  spread  foundations  covering  the  whole 
area  of  the  basement  were  used  for  the  factories  of  Herman 
Behr;&  Co.,  and  W.  H.  Sweeney  Mfg.  Co.,  Brooklyn,  N.  Y. 
Fig.  i59/  shows  the  details  for  the  first-named  factory.  This 
raft  foundation,  which  was  of  the  beam-and-slab  type,  had  a  slab 
thickness  of  i  foot  and  a  beam  thickness  of  3  feet.  The  beams 
formed  continuous  lines  under  the  outer  wall  and  along  the  cen- 
ter line  of  the  columns  lengthwise  of  the  building,  the  column  spac- 
ing being  16  feet  10  inches  longitudinally  and  approximately 
feet  transversely.  These  beams  were  5  feet  wide  under  the 


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FIG.  ISP/. — Spread  Foundation. 


FIG.   i59g. — Spread  Foundation. 


walls  and  6  feet  wide  under  the  columns.  The  intervening 
space  between  beams  was  brought  up  nearly  to  surface  level 
by  a  dirt  fill,  and  a  finished  concrete  floor  was  laid  over  the 
whole  area.  As  shown  in  the  illustration  the  reinforcement  for 
the  1 2  -inch  slab  consisted  of  transverse  bars  i  inch  square, 
spaced  5  inches  on  centers  and  3  inches  from  the  top  of  the  slab. 
The  beams  under  the  columns  were  reinforced  with  eleven 
ij-inch  square  bars  near  the  upper  surface,  the  five  center  bars 
being  carried  through  straight  and  the  six  outside  bars  bent 
down  under  the  column. 

The  foundation  of  the  W.  H.  Sweeney  Mfg.  Company's 
factory  consisted  of  a  slab  over  the  whole  area  surmounted  by 
truncated  pyramidal  slabs  under  all  the  columns  and  a  trape- 


ART.  159 


CONCRETE   SPREAD   FOUNDATIONS 


487 


So  Q.I 


42  Deep 
jij,     °      |    d°-^  °        Rectangular 


-   159^.  —  Reinforced-  concrete  Arch  Foun- 
dation     of   Warehouse    at   418-426  West 


•-//C.5.  Street,   New  York  City. 

Section  6-H.    . 
(Detail  of  Footing) 


SPREAD   FOUNDATIONS  CHAP.  XV 

zoidal-shaped  slab  under  the  wall,  as  shown  in  Fig.  159^.  The 
columns  were  spaced  approximately  16  feet  on  centers  in  both 
directions.  The  column  footings  were  raised  2  feet  6  inches 
above  the  top  of  the  ra'ft  slab  and  the  latter  was  reinforced 
with  six  lines  of  rods  about  ij  feet  on  centers,  and  laid  in  both 
directions  along  the  center  lines  of  the  columns.  Further  rein- 
forcement was  used  in  the  bottom  of  the  slab  under  the  columns 
and  walls,  as  shown  in  the  illustration. 

The  inverted  arch  foundation  of  reinforced  concrete  as  used 
for  a  building  in  New  York  City  presents  an  unusual  type  of 
spread  foundation.  Its  adoption  was  due  to  the  necessity  of 
having  a  very  shallow  foundation.  The  limit  of  depth  fixed 


"""Co/.3  _  Sub  Basement  Floor 


Waterproofing 
SECT-ON  Y-Y 


Waterproofing 

SCCTION  A  A 


FIG.   1592'. — Cellar  Floor  Sections  Showing  Grillage  Beams  and  Reinforced-Concrete 
Girders,    Pope    Building,    Cleveland,    O. 

by  the  architect  was  not  sufficient  for  isolated  reinforced-con- 
crete  footings,  and  as  steel  I-beam  grillages  would  have 
cost  about  25  percent  more,  the  inverted  arch  form  was 
used.  The  arches  ran  in  both  directions  between  columns  as 
shown  in  Fig.  159^.  They  were  12  inches  deep  at  the  crown 
and  42  inches  deep  under  the  cast-iron  column  bases,  and  varied 
from  4  to  5  feet  in  width.  The  reinforcement  consisted  of  J- 
inch  round,  straight,  corrugated  bars  in  the  bottom,  spaced 
6  inches  on  centers,  and  i^-inch  bent  bars  in  the  top,  spaced 
the  same  distance.  All  end  spans  were  made  of  rectangular 
or  T-shaped  concrete  beams,  to  provide  for  the  thrust  in  the 
adjoining  arches.  For  further  details  see  Engineering  News, 
vol.  66,  page  763,  Dec.  28,  1911. 

In  the  foundation  for  the  Pope  Building,  Cleveland,  Ohio,  a 


ART.  159        CONCRETE  SPREAD  FOUNDATIONS  489 

combination  of  a  steel  grillage  and  a  reinforced-concrete  raft 
foundation  was  used.  The  material  upon  which  the  founda- 
tion was  placed  consisted  of  a  few  feet  of  quicksand  overlying 
clay.  As  the  sides  of  the  lot  were  enclosed  by  a  permanent 
steel  cofferdam  extending  well  down  into  the  clay,  the  quick- 
sand was  not  subject  to  outside  disturbance,  and  hence  made  a 
satisfactory  cushion.  A  6-inch  layer  of  concrete  was  first 
spread  over  the  bottom  and  covered  with  tar  and  felt  water- 
proofing, after  which  a  1 6-inch  layer  of  concrete  was  placed 
on  the  waterproofing.  On  this  were  located  the  I-beam  gril- 
lages, as  shown  in  Fig.  1592,  section  A- A  being  taken  at  right 
angles  to  the  street  and  section  Y-Y  parallel  with  the  street. 
The  grillages  were  made  of  two  tiers  of  24-inch  I-beams,  each 
supporting  a  single  column.  In  all  the  intermediate  spaces 
the  concrete  floor  slab  was  reinforced  with  rods,  thus  providing 
for  the  distribution  of  the  column  loads  over  the  entire  bottom. 


CHAPTER  XVI 
UNDERPINNING  BUILDINGS 

ART.  1 60.     NEEDLE-BEAM  UNDERPINNING 

The  technical  term  underpinning  is  used  to  denote  the  placing 
of  new  foundations  or  supports  under  existing  structures.  As 
an  engineering  art  and  science  this  work  has  been  developed 
almost  entirely  in  a  few  large  cities,  notably  New  York,  Chicago 
and  Boston.  In  New  York  the  subways  and  the  modern 
'sky-scraper/  with  its  foundations  carried  far  below  those  of 
surrounding  structures,  have  compelled  the  placing  of  new  and 
deeper  foundations  for  many  buildings.  Some  of  these  under- 
pinned buildings  have  wall  loads  as  high  as  45  tons  per  linear 
foot  and  column  loads  of  300  tons  or  more.  The  underpinning 
of  such  heavy  buildings  requires  great  skill  and  care,  for  it  must 
be  done  in  such  a  manner  that  no  settlement  occurs;  with 
the  mechanical  equipment  of  the  modern  office  building,  such 
as  elevators,  motors,  engines,  etc.,  a  very  slight  differential 
settlement  often  causes  trouble.  Moreover,  the  work  must 
often  be  done  hastily  and  in  a  limited  space. 

The  two  general  methods  of  underpinning  are :  First,  the  use 
of  needle-beams  to  support  the  structure  temporarily,  after 
which  the  old  foundations  are  removed  and  new  ones  placed; 
and  second,  the  use  of  vertical  cylinders  (without  temporarily 
supporting  the  structure)  in  the  plane  of  and  under  the  walls, 
carried  down  to  solid  bearing. 

The  needle-beam  method  of  underpinning  may  be  called 
the  indirect  method  since  the  function  of  the  needle-beams  is 
merely  to  take  the  loads  temporarily  from  the  old  foundation 
to  permit  removing  the  latter  and  the  building  of  new  founda- 
tions. This  method  is  the  older  and  more  widely  used,  being 
universally  applied  where  the  new  foundation  is  of  a  simple 
type  and  not  carried  to  a  great  depth. 

490 


To      O 

AND     TO     $100     ON 
OVERDUE.  °N 


REC.  CIR. 


SEVENTH     DAY 


YC  13427 


3 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


